Roger Federer saw one Olympic title slip away at Wimbledon today but remained on track for another as he moved into the quarter-finals of the men's singles with victory over Denis Istomin.

Federer did not have things all his own way against the Uzbek, who had the chance to break to lead 6-5 in the first set, but he could not take it and did not get another one, the world number one coming through 7-5 6-3.

Federer then returned to the court alongside Stanislas Wawrinka, the man with whom he won an emotional and surprise doubles gold in Beijing.

They raced through the first set against experienced Israeli pair Jonathan Erlich and Andy Ram but could not close it out and were beaten 1-6 7-6 (7/5) 6-3.

Federer said: "I'm disappointed but overall I'm feeling good. Best-of-three-set tennis is very fast. There is no room for error really. In the doubles, even more so. I think we played two good doubles matches and I'm playing well in the singles."

Federer faces the definition of a tall task in the quarter-finals, though, as he takes on 6ft 9in American John Isner, who will be looking to write new headlines at Wimbledon.

Isner has already beaten Federer this season in the Davis Cup, and the Swiss star said: "I know it's going to be a tough one for me.

"Obviously John has got one of the best serves on tour, if not the best, and that obviously makes it complicated getting into rallies and into a rhythm. That's grass-court tennis."

Isner ended the hopes of Serbian seventh seed Janko Tipsarevic today, surviving a titanic tie-break to win 7-5 7-6 (16/14). Tipsarevic finished the match on a double fault, which left him fuming.

He said: "The line umpire said 'in' and the chair umpire overruled on the match point, which is unbelievable in my opinion, but there was no HawkEye so I couldn't see how the ball was."

Second seed Novak Djokovic was pushed all the way by a rejuvenated Lleyton Hewitt, but the Serb dug in and eventually triumphed 4-6 7-5 6-1 in a high-quality match.

Hewitt, the champion at Wimbledon 10 years ago, has been dogged by injuries over the past few seasons and may have retired but for radical toe surgery earlier this year.

He needed a wild card to get into the tournament after dropping to 159th in the world but has more than justified it with two victories and a superb performance today.

Hewitt broke in the ninth game and clinched the first set with a fizzing forehand winner, and he retrieved a break in the second set to level at 5-5.

But he could not force a tie-break and in the decider Djokovic's weight of shot was just too much.

The world number two was not surprised by Hewitt's performance and believes the Australian can get back to the top of the game if he avoids further injuries.

Djokovic said: "I think he plays his best tennis on grass. He's a former world number one. I have a lot of respect for him. He's a great competitor.

"He has shown again that he's a player that loves to play on a big stage. He loves to play big matches, especially for his country. We all know how much effort he puts into playing the Davis Cup, when he wears the Australian shirt.

"He deserves to be top 20. With the game he showed today, he can beat anybody on a given day, especially on the fast surfaces."

Hewitt was proud of his performance and has taken great encouragement from the level he has found at the Olympics.

The 31-year-old said: "I threw everything I had at him and had my chances. I served really well for the most part but he started returning very well. It's a game of inches out there.

"When you go through a whole lot of surgery you can't expect to pick up your ball skills where you left off. It's going to take time. The last couple of weeks have given me a great deal of confidence."

Things certainly do not get any easier for Djokovic, who next faces a blockbuster quarter-final against fifth seed Jo-Wilfried Tsonga.

The Frenchman recovered impressively from his titanic battle against Milos Raonic yesterday, which went to 25-23 in the third set, to defeat Spain's Feliciano Lopez 7-6 (7/5) 6-4.

There was a late-night shock as Japan's Kei Nishikori defeated fourth seed and Wimbledon quarter-finalist David Ferrer of Spain 6-0 3-6 6-4.

Nishikori led 5-4 on serve in the third set when the decision was taken to move the match under the roof on Centre Court, and the 15th seed won four of the five points played to secure arguably the best result of his career.

In the quarter-finals he will face eighth seed Juan Martin Del Potro from Argentina, who defeated France's Gilles Simon 6-1 4-6 6-3, while 11th seed Nicolas Almagro was a 7-5 6-3 winner over Steve Darcis and next meets Andy Murray.