Inclusion Criteria:

          -  Written informed consent provided.

          -  Females 18 years old and greater (at the time of written consent)

          -  Histologically or cytologically confirmed diagnosis of advanced or metastatic
             adenocarcinoma of the breast.

          -  Documentation of estrogen receptor (ER)-positive and/or progesterone receptor
             (PR)-positive tumor (>=1% positive stained tumor cell nuclei) based on local testing
             of the most recent tumor biopsy, using an assay consistent with local standards.

          -  Documentation of HER2-negative tumor based on local testing of the most recent tumor
             biopsy as per most recent American Society of Clinical Oncology (ASCO)/College of
             American Pathologists (CAP) guidelines. At the time of writing, HER2-negative tumor is
             defined as immunohistochemistry (IHC) score of 0 or 1+, or negative by in situ
             hybridization defined as a HER2/chromosome enumeration probe 17 (CEP17) ratio <2 or
             for single probe assessment of an average HER2 copy number <4.

          -  Provision of mandatory screening fresh tumor biopsy sample during the screening
             period: a. Screening biopsy can be waived if a biopsy was collected within 3 months
             prior to first dose of study drug and was collected after the last anti-cancer
             treatment before coming into this study; b. Participants with inaccessible site of
             biopsy or who have a significant medical risk of obtaining the biopsy should be
             discussed with the Medical Monitor if they can qualify; c. Bone biopsies are not
             acceptable. Biopsies should be obtained from bone with metastatic soft-tissue
             component. Participants with bone only disease may be enrolled upon review by Medical
             Monitor.

          -  History of prior therapy that satisfies one of the following criteria: a. Aromatase
             inhibitor (AI) failures: Disease that relapsed during treatment or within 12 months of
             completion of adjuvant therapy with an AI, OR disease that progressed during treatment
             with an AI for advanced/metastatic disease. Prior ovarian suppression and/or tamoxifen
             are allowed as long as other criteria are met; b. Cyclin-dependent kinase 4/6 (CDK4/6)
             inhibitor plus AI failures: Disease that progressed on a CDK4/6 inhibitor plus AI, for
             advanced/metastatic disease with a minimum duration of treatment of 12 months (>=12
             months) with CDK4/6 inhibitor plus AI. Participants with either measurable disease or
             bone only disease are allowed. Prior ovarian suppression and/or tamoxifen are allowed
             as long as other criteria are met.

          -  Documented progression on last line of systemic anti-cancer therapy with CDK4/6
             inhibitor plus AI is required.

          -  Any menopausal status.

          -  Measurable disease by Response Evaluation Criteria in Solid Tumors (RECIST) 1.1
             criteria is required except for participants with bone only disease.

          -  All prior treatment- related toxicities must be National Cancer Institute-Common
             Terminology Criteria for Adverse Events (NCI-CTCAE) version 4 <=Grade 1 (except
             alopecia (permitted at any grade) and peripheral neuropathy (permitted at <=Grade 2)
             at the time of treatment allocation.

          -  Eastern Cooperative Oncology Group (ECOG) Performance Status score of 0 to 1.

          -  Adequate organ function.

          -  Able to swallow and retain orally administered medication.

          -  A female participant is eligible to participate if she is of: i) Non-childbearing
             potential. ii) Child-bearing potential and agrees to use one of the contraception
             methods. iii) Negative serum pregnancy test <=7 days prior to first study drug dose.
             iv) Female participants who are lactating must discontinue nursing prior to the first
             dose of study treatment and must refrain from nursing throughout the treatment period
             and for at least 28 days following the last dose of study treatment.

        Exclusion Criteria:

          -  Prior therapy with any Bromodomain and extra-terminal (BET) inhibitor, any selective
             estrogen receptor degrader (SERD) including fulvestrant, or inhibitors of the
             Phosphoinositide-3-kinase (PI3K)/ serine/threonine-specific protein kinase
             (AKT)/Mammalian Target of Rapamycin (mTOR) pathway.

          -  Prior therapy with more than one line of cytotoxic chemotherapy following diagnosis of
             advanced/metastatic disease.

          -  More than or equal to 3 lines of systemic anti-cancer therapy in the advanced or
             metastatic setting.

          -  Recent prior therapy, defined as: a. Any investigational or approved non-biologic
             anti-cancer drug within 14 days or five half-life (whichever is greater) prior to the
             first dose of GSK525762 and fulvestrant. b. Any nitrosoureas or mitomycin C within 42
             days prior to the first dose of GSK525762 and fulvestrant c. Any anti-cancer biologic
             agents within 42 days prior to the first dose of GSK525762 and fulvestrant. d. Any
             radiotherapy within 14 days prior to the first dose of GSK525762 and fulvestrant. If
             the participant received radiotherapy <90 days prior to study treatment, the
             irradiated lesion cannot be the only lesion used for evaluating response.

             e. Any major surgery within 28 days prior to the first dose of GSK525762 and
             fulvestrant

          -  Concomitant active malignancy other than HR+/HER2- breast cancer

          -  Therapeutic-dose anticoagulation (e.g., warfarin, low-molecular weight heparin [LMWH],
             or novel oral anticoagulants) must be discontinued and coagulation parameters must be
             normalized prior to the first dose of GSK525762 and fulvestrant. Prophylactic
             anticoagulation, with low doses (per standard practice) of agents such as LMWH, direct
             thrombin inhibitors, or factor Xa inhibitors is permitted.

          -  Current use of a prohibited medication or planned use of any forbidden medications
             during treatment with GSK525762 and fulvestrant. This includes medications with
             significant risk of Torsades de pointes as well as those that are potent inducers or
             inhibitors of Cytochrome P3A4 (CYP3A4) enzymes.

          -  Evidence of severe or uncontrolled systemic diseases (e.g., unstable or uncompensated
             respiratory, hepatic, renal, cardiac disease, or clinically significant bleeding
             episodes). Any serious and/or unstable pre-existing medical (aside from malignancy),
             psychiatric disorder, or other conditions that could interfere with participant's
             safety, obtaining informed consent or compliance to the study procedures, in the
             opinion of the Investigator. a) Systolic blood pressure higher than 150 millimeters of
             mercury (mmHg) or diastolic blood pressure higher than 90 mmHg found on 2 separate
             occasions separated by 1 week, despite adequate therapy, will be defined as
             uncontrolled hypertension. b) Uncontrolled diabetes mellitus (despite therapeutic;
             compliance to intervention) as defined by a hemoglobin A1c (HbA1c) level more than 8%
             and/or occurrence of more than two episodes of ketoacidosis in the 12 months prior to
             the first dose of study drug.

          -  Participants with advanced/metastatic, symptomatic, visceral spread, that are at risk
             of life-threatening complications in the short term including participants with
             massive uncontrolled effusions (pleural, pericardial, peritoneal), pulmonary
             lymphangitis, and over 50 percent (%) of liver involvement in metastases.

          -  Symptomatic or untreated leptomeningeal or brain metastases or spinal cord
             compression.

          -  Cardiac abnormalities as evidenced by any of the following: Baseline QT interval
             corrected by Fridericia's formula (QTcF) interval >=480 milliseconds (msec);
             Clinically significant conduction abnormalities or arrhythmias; Presence of cardiac
             pacemaker or defibrillator with a paced ventricular rhythm limiting electrocardiogram
             analysis; History or evidence of current >=Class II congestive heart failure as
             defined by New York Heart Association (NYHA); History of acute coronary syndromes
             (including unstable angina and myocardial infarction), coronary angioplasty, or
             stenting within the past 3 months. Participants with a history of stent placement
             requiring ongoing antithrombotic therapy (e.g., clopidogrel, prasugrel) will not be
             permitted to enroll; Clinically significant cardiomegaly, ventricular hypertrophy, or
             cardiomyopathy.

          -  Current active liver or biliary disease (with the exception of Gilbert's syndrome or
             asymptomatic gallstones, liver metastases or otherwise stable chronic liver disease
             per investigator assessment).

          -  Presence of hepatitis B surface antigen (HBsAg) or positive hepatitis C antibody test
             result at screening.

          -  History of known human immunodeficiency virus (HIV) infection.

          -  Any serious known immediate or delayed hypersensitivity reaction(s) to GSK525762 or
             fulvestrant, or idiosyncrasy to drugs chemically related to the investigational drugs.

          -  Hemoptysis >1 teaspoon in 24 hours within the last 28 days.

          -  Concurrent use of non-steroidal anti-inflammatory drugs (NSAIDs) (except for cases
             where NSAIDs provide benefit over other analgesics and in these cases, consideration
             should be given to the prophylactic administration of a proton pump inhibitor) and
             high dose aspirin (allowed up to <=100 milligrams orally daily).

          -  Participants with history of known bleeding disorder(s) including clinically
             significant hemorrhage (e.g., gastrointestinal, neurologic), within the past 6 months.

          -  Any clinically significant gastrointestinal abnormalities that may alter absorption,
             such as malabsorption syndrome, chronic gastrointestinal disease, or major resection
             of the stomach and/or bowels that could preclude adequate absorption of the study
             medication.