Question: who has won the most masters titles in tennis
[{'answer': 'Rafael Nadal', 'provenance': [{'wikipedia_id': '30872553', 'title': 'ATP Tour Masters 1000', 'start_paragraph_id': 3, 'start_character': 12, 'end_paragraph_id': 3, 'end_character': 24, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'Novak Djokovic', 'provenance': [{'wikipedia_id': '30872553', 'title': 'ATP Tour Masters 1000', 'start_paragraph_id': 3, 'start_character': 110, 'end_paragraph_id': 3, 'end_character': 124, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]