Roger Federer's record 4-1/2 year reign as world number one could end this week after he was beaten 7-6, 4-6, 7-6 by Croatian Ivo Karlovic in the third round of the Cincinnati Masters on Thursday.
Spaniard Rafael Nadal, who beat Federer in this year's French Open and Wimbledon finals, will take over from the Swiss as the world's top ranked player if he wins the tournament on Sunday.
The 2.08-metre tall Karlovic, who has hit more aces and won more service games than any other player on the ATP Tour this year, applied constant pressure.
Karlovic, who hit 22 aces and many more service winners, has also played more tiebreaks than anyone else this season, the potency of his delivery frequently reducing each set to a shootout over a dozen or so points at the end.
This was about the worst scenario for an already under-pressure Federer, who is still trying to recover from the biggest emotional setback of his career - the loss of his Wimbledon title to Nadal three and a half weeks ago - and whose confidence is at low levels.
"It's not easy coming from slower conditions to fast ones," Federer told reporters. "But I have played Ivo on all surfaces and he actually gets a greater play on the slower ones.
"But what are you going to do today if he serves like that? Maybe if I got the first set I would be sitting here and it would be different, but that's not the case."
Last week Federer lost his opening match in Toronto to unseeded Frenchman Gilles Simon and here he almost went out in his opener again, surviving against Robbie Ginepri only after the American was serving for the match.
When Karlovic won his points it was usually with sharp staccato rallies, while Federer often had to play the rallies out to maintain parity, thereby increasing the pressure on him to make mistakes as the match went on.
In the opening-set tiebreak it all went with serve till 6-6 when Federer topspun a drive which was called in, only for it to be changed after Karlovic appealed to the Hawkeye replay system. Karlovic then hit a winning forehand drive on the next point.
Federer chiselled out one break of serve in the fifth game of the second set but in the deciding third-set tiebreak, Federer suffered a chronic piece of ill-fortune.
On his opening service point, a forehand drive struck the top of the net cord and leapt the full length of the court to land long. Karlovic clung to that advantage and ended the match with a service winner on the line.