Juan Martin Del Potro of Argentina celebrates defeating Fernando Verdasco of Spain in their match at the Indian Wells ATP tennis tournament in Indian Wells, California March 13, 2012. [Photo\Agencies]

INDIAN WELLS, California -  Fifth-seeded Spaniard David Ferrer was ousted from the Indian Wells ATP tournament on Tuesday as Juan Martin Del Potro survived a shaky second set before moving into the fourth round.

Ferrer was upset 6-4 6-3 by Uzbekistan's Denis Istomin while big-serving Del Potro of Argentina advanced with a 6-2 7-6 victory over Spaniard Fernando Verdasco at the Indian Wells Tennis Garden.

A winner of three ATP titles this year, Ferrer had lost only once in 20 matches this season before struggling badly on his second serve as he was outplayed by the 51st-ranked Istomin.

"I didn't play a good match even though I don't want to take credit from him," the Spaniard tweeted in his native language. "He was the better player today and played very well, a fair winner."

Claycourt specialist Ferrer, a former world number four, had won titles in Auckland, Buenos Aires and Acapulco this year.

Istomin, who broke the out-of-sorts Spaniard once in the opening set and three times in the second, will next meet ninth seed Del Potro, who saved six set points before booking his place in the fourth round.

The six-foot six-inch Argentinian, who won his first grand slam title at the 2009 US Open, breezed through his opening set in 39 minutes before struggling against the Spanish left-hander's fluent ground strokes.

Del Potro saved four set points on his own serve in a marathon 10th game, and two more in the tiebreak, before sealing victory when a Verdasco backhand flew long.

Victory Delight

The six-foot six-inch South American clenched his fists in delight before making a sign of the cross on his chest and looking skywards in relief as the stadium court crowd erupted in applause.

"I am still very nervous from that moment when Fernando played much better than me in the second set," the 23-year-old Del Potro said in a courtside interview.

"Verdasco is a really difficult opponent, especially in the early rounds of the big tournaments. He's very, very tough and very dangerous. Sometimes you are lucky to win these kinds of matches."

Sixth seed Jo-Wilfried Tsonga of France battled back to beat Czech Radek Stepanek 6-7 6-3 6-2 while Brazilian Thomaz Bellucci was gifted a walkover when Nikolay Davydenko withdrew from their match due to illness.

It was not certain whether Russian Davydenko had succumbed to the viral infection which has already forced eight players to pull out of Indian Wells, tournament officials said.

Bellucci will next play either three-times champion Roger Federer or Canadian Milos Raonic who were scheduled to play in Tuesday's evening session.

In other third-round matches, twice winner and second seed Rafa Nadal was playing fellow Spaniard Marcel Granollers while David Nalbandian of Argentina was scheduled to meet Serb Janko Tipsarevic.