Question: who has won the most masters tennis titles
[{'answer': 'Rafael Nadal', 'provenance': [{'wikipedia_id': '32216998', 'title': 'Tennis Masters Series records and statistics', 'start_paragraph_id': 3, 'start_character': 317, 'end_paragraph_id': 3, 'end_character': 329, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '32216998', 'title': 'Tennis Masters Series records and statistics', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 508, 'bleu_score': 0.848831992167387, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 16558539629523284542}, 'section': 'Section::::Abstract.'}]}]