Question: who has won the 2017 mens singles mutua madrid open tennis
[{'answer': 'Rafael Nadal', 'provenance': [{'wikipedia_id': '53944110', 'title': "2017 Mutua Madrid Open – Men's Singles", 'start_paragraph_id': 1, 'start_character': 73, 'end_paragraph_id': 1, 'end_character': 85, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '52628704', 'title': '2016 Mubadala World Tennis Championship (December)  Singles', 'section': 'Section::::Abstract.', 'start_paragraph_id': 1, 'end_paragraph_id': 1, 'meta': {'evidence_span': ["Rafael Nadal was the competition's defending champion and successfully defended his title", "Rafael Nadal was the competition's defending champion and successfully defended his title, defeating David Goffin in the final, 64, 76."]}}]}]