Roger Federer's grip loosened on his long-time number-one ranking as the Swiss lost to Croatian Ivo Karlovic 7-6 (8-6), 4-6, 7-6 (5-7) on Thursday at the Cincinnati Masters, dpa reported.
If Paris and Wimbledon winner Rafael Nadal can lift the title on Sunday, he will take over Monday as the top man in the game for the first time.
Federer, with only two titles this season, floated a backhand return long to lose the contest after beating the huge Croatian in six previous matches.
Nadal faced a later third-round match against Tommy Haas. With an upset, the German could save Federer's ranking for at least another week.