The third-round loss means Nadal can now usurp the Swiss at the top of the rankings by winning the title on Sunday.

Since an epic five-set loss to the second-ranked Nadal at Wimbledon, Federer has lost in the second round in Toronto and now failed to make the quarter-finals at a tournament he won last year.

Federer has been ranked No.1 for a record 235 consecutive weeks, a streak that will end if Nadal - who was scheduled to play Tommy Haas tonight - adds to his run of five consecutive tournament titles.

Federer needed three sets to get through his first match in Cincinnati, then fell behind again today to the hard-serving Karlovic.

Federer has won all six times they played previously, taking 13 of their 15 sets, but many of them were close.

Nine of those 15 sets ended with tiebreakers.

He got the worst of the most recent ones. When his backhand return sailed long to end the match, Karlovic plopped on his back and raised his arms.

AFP/AP