Roger Federer's reign as world number one is under serious threat after he lost to 16th seed Ivo Karlovic in the third round of the Cincinnati Masters.
The Swiss' 6-7 6-4 6-7 defeat to the big-serving Croatian means Rafael Nadal could become world number one should the Spaniard win the tournament.
Nadal, ranked second in the world, progressed to the quarter-finals with a 6-4 7-6 defeat of Tommy Haas.
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