A Study of Venetoclax in Participants With Relapsed or Refractory Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma

* Participant must have a diagnosis of relapsed or refractory chronic lymphocytic leukemia (CLL)/Small Lymphocytic Lymphoma (SLL) that meets 2008 Modified International Workshop for Chronic Lymphocytic Leukemia (iwCLL) National Cancer Institute-Working Group (NCI-WG) Guidelines and the following: * Participant must have an indication for treatment according to the 2008 Modified iwCLL NCI-WG Guidelines. * SLL participant must have measurable disease (B-lymphocytosis greater than 5 × 10\^9/L or an enlarged lymph node(s) (Longest Diameter (LDi) \> 1.5 cm at baseline) or hepatomegaly or splenomegaly due to CLL). * SLL participant must have presence of lymphadenopathy and absence of cytopenias caused by a clonal marrow infiltrate. * Participant must have relapsed or refractory CLL/SLL after receiving at least one prior line of therapy. * Participants (in Cohort 1) must have 17p deletion, assessed by a central laboratory. * Participants (in Cohort 2) must meet both of the following: * Relapsed/refractory disease to B-Cell Receptor Signaling Pathway Inhibitor (BCRI) treatment; * And either of the following: (a) relapsed/refractory disease to chemoimmunotherapy (CIT), or (b) ineligible to receive CIT, defined as having known 17p deletion or TP53 mutation, or Cumulative Illness Rating Scale (CIRS) \>6 or calculated creatinine clearance \<70 mL/min, or participants in whom the investigator evaluated that the use of CIT was inappropriate. * Participant must have an Eastern Cooperative Oncology Group (ECOG) performance score of less than or equal to 2. * Participant must have adequate bone marrow function, coagulation profile, renal, and hepatic function, per laboratory reference range at Screening. * No known active severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.

* Participant has undergone an allogeneic stem cell transplant. * Participant has developed Richter's transformation confirmed by biopsy. * Participant has prolymphocytic leukemia. * Participant has active and uncontrolled autoimmune cytopenias (for 2 weeks prior to screening), including autoimmune hemolytic anemia (AIHA) and idiopathic thrombocytopenic purpura (ITP). * Participant has previously received venetoclax or other BCL-2 inhibitors. * Participant is known to be positive for Human Immunodeficiency Virus (HIV). * Participant has received a biologic agent for anti-neoplastic intent within 30 days prior to the first dose of study drug. * Participant has received any of the following within 14 days or 5 half-lives (whichever is shorter) prior to the first dose of venetoclax, or has not recovered to less than Common Toxicity Criteria for Adverse Events (CTCAE) grade 2 clinically significant adverse effect(s)/toxicity(s) of the previous therapy: * Any anti-cancer therapy including chemotherapy, immunotherapy, radiotherapy or targeted small molecule agents. * Investigational therapy, including targeted small molecule agents. * Participant has known allergy to both xanthine oxidase inhibitors and rasburicase.