Inclusion Criteria:

          -  For acute myeloid leukemia (AML) participants: AML according to World Health
             Organization (WHO) 2016 criteria and fulfilling all of the following criteria:(a) more
             than or equal to (>=) 75 years of age, or younger participants who are not eligible
             for or not willing to receive an intensive treatment (including stem cell
             transplantation) with a curative intent and (b) previously untreated AML (except:
             emergency leukapheresis, low dose of cytarabine and/or hydroxyurea during the
             screening phase to control hyperleukocytosis but must be discontinued at least one day
             prior to start of cusatuzumab [Part 1] or azacitidine [Part 2]). All trans retinoic
             acid (ATRA) treatment for presumed acute promyelocytic leukemia (APL) is permitted but
             must be discontinued at least 1 day prior to the start of azacitidine

          -  For Myelodysplastic Syndrome (MDS) participants (only for Part 2): MDS according to
             WHO 2016 criteria and fulfilling all of the following criteria: (a) Not eligible for
             or not willing to receive allogenic stem cell transplantation,(b) very high or
             high-risk MDS according to Revised International Prognostic Scoring System (IPSS-R)
             and (c) previously untreated MDS (except: transfusion and/or cytokine therapy
             including erythropoietin)

          -  Eastern Cooperative Oncology Group (ECOG) performance status score of 0, 1 or 2

          -  Must sign an informed consent form (ICF) indicating that he or she understands the
             purpose of, and procedures required for, the study and is willing to participate in
             the study

          -  A woman of childbearing potential must have a negative highly sensitive serum (beta
             human chorionic gonadotropin [beta hCG]) or urine pregnancy at screening

        Exclusion Criteria:

          -  Acute promyelocytic leukemia (APL) with t (15;17), or its molecular equivalent
             promyelocytic leukemia retinoic acid receptor (PML RAR alpha)

          -  Leukemic involvement or clinical symptoms of leukemic involvement of the central
             nervous system

          -  Known allergies, hypersensitivity, or intolerance to cusatuzumab or azacitidine or its
             excipients (example, mannitol, an excipient of azacitidine)

          -  Prior treatment with a hypomethylating agent for treatment of AML or MDS

          -  A diagnosis of other malignancy that requires concurrent nonsurgical treatment