Argentina's Juan Martin del Potro scored a 6-1, 4-6, 6-3 victory Wednesday over France's Gilles Simon to advance to the quarterfinals of the Olympics tennis men's singles.

Del Potro, the last remaining Latin American player in the men's singles competition, lost his first set of the tournament but rebounded nicely for the victory and on Thursday will take on surprise quarterfinalist Kei Nishikori of Japan, who upset Spain's David Ferrer 6-0, 3-6, 6-4 on Wednesday.

"For me this is special. I don't want to take a measure of this because I'm still in the tournament. I know I'm doing well and I want to keep on this path. The matches are tough but I'm doing well," Del Potro said.

Ferrer's loss, meanwhile, was the biggest surprise thus far on the men's side, especially considering the Spaniard's red-hot form in recent weeks and quarterfinal showing last month at Wimbledon.

The match began on an outside court before being moved to the covered center court because of darkness, but Ferrer refused to blame that shift in venue for the loss.

"In the end, he was better than me and only the weak look for an excuse," the world No. 4 said. "He started off great. He was more aggressive and I lacked concentration at the end, in the final game."

Ferrer's exit leaves Nicolas Almagro, a 7-5, 6-3 winner Wednesday over Belgium's Steve Darcis, as the only remaining Spanish player in the men's singles.

In other third-round action Wednesday, world No. 1 Roger Federer continued his strong run of form with a 7-5, 6-3 win over Uzbekistan's Denis Istomin.

The Swiss, back on the grass of Wimbledon just a few weeks after winning his record-tying seventh title at the All England Club, on Thursday will take on big-serving American John Isner, who edged Serbia's Janko Tipsarevic 7-5, 7-6.

The winner of that match will face either Del Potro or Nishikori.

In the bottom half of the draw, Almagro will square off Thursday against Britain's Andy Murray - a 4-6, 6-1, 6-4 winner Wednesday over Cypriot Marcos Baghdatis - in one quarterfinal.

The last quarterfinal will pit world No. 2 Novak Djokovic of Serbia - who survived a scare Wednesday from Australia's Lleyton Hewitt before winning 4-6, 7-5, 6-1 - against France's Jo-Wilfried Tsonga, a 7-6, 6-4 victor over Spaniard Feliciano Lopez.

In men's doubles action Wednesday, Brazil's Marcelo Melo and Bruno Soares finished off a marathon victory over the Czech Republic's Tomas Berdych and Radek Stepanek 1-6, 6-4, 24-22. The match, which began on Tuesday, finally concluded after the players spent four hours and 21 minutes on court.

Del Potro and partner Gisela Dulko also got off to a good start Wednesday in the mixed doubles competition, knocking off the Russian team of Mikhail Youzhny and Elena Vesnina 6-3, 7-5. EFE