Roger Federer's grip on the world number one spot could be loosening after the Swiss crashed out of the Cincinnati Masters at the hands of Ivo Karlovic last night.
The Swiss top seed found the big-serving Croatian too much to handle as he slumped to a 7-6 (8/6) 4-6 7-6 (7/5) defeat in the third round.
Federer's failure to reach the quarter-finals could prove even more costly as he will now lose his long-held number one ranking if Rafael Nadal wins the tournament.
The Spaniard, recent winner of Wimbledon and the French Open - where he beat Federer in both finals, overcame Tommy Haas.
Federer has been the world's top-ranked player for a record 235 consecutive weeks.
He fought his way back into a tight contest against Karlovic by taking the second set but neither player could gain the upper hand in the third.
It needed a tie-break to settle the match and Karlovic claimed the first two points of it - the second of which came on Federer's serve, allowing him to carve out three match points.
And although the Croatian pushed the first two long, Federer then hit a long return of his own to hand the match to Karlovic.