World number one and defending champion Novak Djokovic was knocked out of the Madrid Masters on Friday, losing 7-6 (7/2), 6-3 to Serb compatriot Janko Tipsarevic.
Djokovic's quarter-final defeat comes a day after world number two Rafael Nadal was defeated by fellow Spaniard Fernando Verdasco in the third round.
Roger Federer, the 16-time Grand Slam champion, who faces Spain's David Ferrer later Friday, could move to second in the world behind Djokovic should he win the title on Sunday.
Tipsarevic earned only his second career win against Djokovic, who beat him last February in a quarter-final in Dubai.
He claimed the 61-minute opening set on a tiebreaker, then remained collected as Djokovic saved three match points in the eighth game of the second set before losing a game later as Tipsarevic came through on his fourth opportunity.