Thibault Hutin, a 26-year-old French equestrian show jumper, told Swiss newspaper SonntagsBlick he knows of at least two times Hingis, 32, has cheated on him.
“Martina has a very personal conception of morality," said Hutin. "She has always been like that; I think she has always been unfaithful to her boyfriends."
The news comes as the so-called "Swiss Miss" is set to be inducted into the International Tennis Hall of Fame Saturday in Newport, R.I.
Hingis was ranked No. 1 in women's tennis for 209 weeks starting when she was 15. She won three Grand Slam titles in 1997.
"Being inducted into the International Tennis Hall of Fame is a tremendous honor," Hingis said at the time of the class announcement. "It is truly a privilege to be part of such an exclusive group of tennis icons. I am looking forward to the enshrinement weekend in Newport and to being welcomed in by the other Hall of Famers."