Who has won more head-to-head tennis matches between each other, Serena Williams or Maria Sharapova?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q1860', 'id': 0, 'label': 'English'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q11666', 'id': 1, 'label': 'Maria Sharapova'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q854464', 'id': 2, 'label': 'Serena'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q11459', 'id': 3, 'label': 'Serena Williams'}, {'type': 'INTERNAL', 'name_': 'Q16275104', 'id': 4, 'label': 'Best Female Tennis Player ESPY Award'}, {'type': 'INTERNAL', 'name_': 'Q16218497', 'id': 5, 'label': 'Alexander Gilkes'}], 'links': [{'name_': 'P407', 'source': 2, 'target': 0, 'label': 'language of work or name'}, {'name_': 'P1412', 'source': 3, 'target': 0, 'label': 'languages spoken, written or signed'}, {'name_': 'P735', 'source': 3, 'target': 2, 'label': 'given name'}, {'name_': 'P1411', 'source': 3, 'target': 4, 'label': 'nominated for'}, {'name_': 'P1346', 'source': 4, 'target': 1, 'label': 'winner'}, {'name_': 'P1346', 'source': 4, 'target': 3, 'label': 'winner'}, {'name_': 'P1412', 'source': 5, 'target': 0, 'label': 'languages spoken, written or signed'}, {'name_': 'P451', 'source': 5, 'target': 1, 'label': 'unmarried partner'}]}