Who's won more head-to-head tennis matches between each other, Novak Djokovic or Roger Federer?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'QUESTIONS_ENTITY', 'name_': 'Q1426', 'id': 0, 'label': 'Roger Federer'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q5812', 'id': 1, 'label': 'Novak Djokovic'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q757054', 'id': 2, 'label': "tennis at the 2008 Summer Olympics – men's doubles"}], 'links': [{'name_': 'P3342', 'source': 1, 'target': 0, 'label': 'significant person'}, {'name_': 'P1344', 'source': 1, 'target': 2, 'label': 'participant in'}, {'name_': 'P1346', 'source': 2, 'target': 0, 'label': 'winner'}]}