The Centre Court crowd, buzzing with casual conversation during a changeover, suddenly went silent when the chair umpire uttered words rarely heard at Wimbledon, or anywhere else.

"Ladies and gentlemen," he said, "Mr. Federer is taking an off-the-court medical timeout."

Roger Federer has seldom been slowed by health issues, but he briefly left the court Monday because of a back injury and had spectators wondering whether he would return.

After an eight-minute delay, Federer resumed whacking winners and went on to beat frequent foil Xavier Malisse 7-6 (1), 6-1, 4-6, 6-3. The 16-time Grand Slam champion reached his 33rd consecutive major quarterfinal, extending his Open era record.

On Wednesday he'll play No. 26-seeded Mikhail Youzhny, who edged Denis Istomin 6-3, 5-7, 6-4, 6-7 (5), 7-5.

Federer improved to 10-1 against Malisse. He's 13-0 against Youzhny.

"We know what to expect, both of us," Federer said. "I hope to recover and play a good match against him."

Defending champion Novak Djokovic beat fellow Serb and Olympic doubles partner Viktor Troicki 6-3, 6-1, 6-3. Three matches were suspended because of rain and darkness, and the other two fourth-round matches never started and will begin Tuesday.

Federer's back began bothering him early in his match. He blamed the cool, windy weather and the lingering effects of an arduous five-set win over Julien Benneteau three days earlier.

Federer's serve lacked its usual speed, but his play seemed otherwise unaffected by the bothersome back. An hour after the victory, he said he already felt much better.

"Honestly I'm not too worried," he said. "I've had bad backs over the years. They go as quick as they came. But, of course, I have to keep an eye on it now. Two good nights' sleeps, and I'll be 100 percent on Wednesday. I'm pretty convinced, otherwise I wouldn't have been able to pull out the match the way I did today."

While the six-time Wimbledon champion has undoubtedly felt a back twinge or other discomfort on occasion, he has also played in 51 consecutive Grand Slam tournaments, a testament to his durability. It's unusual to see him require attention from a trainer, but that was the case midway through the first set.

He sat in a changeover chair, leading but ailing, when the trainer came out to check him. After a brief conversation, they left the court together.

"I felt the back going the beginning of the first set," Federer said. "I asked for the trainer to come out to just talk about it. I decided to have treatment inside."

Federer returned to cheers and gave Malisse a wave of apology for the delay. He then eased any concern about his condition by playing a succession of spectacular points to win the set.

With Malisse serving at 6-5, Federer rocketed a backhand, then sprinted forward to scoop a delicate forehand cross-court for a winner. He hit a backhand winner to reach break point and put away a deft backhand volley to break for 6-all.

From 1-1 in the ensuing tiebreaker, Federer took the set by sweeping six consecutive points, the last a drop shot for a winner as Malisse slipped behind the baseline and went sprawling.

The Belgian was down but not out. He took the third set and led 2-love in the fourth before Federer began another surge, sweeping the next five games.

Among those monitoring Federer's progress was the top-ranked Djokovic, a potential opponent in the semifinals.

"He's 30 years old now, but he's still hungry to be No. 1 of the world," Djokovic said. "He's definitely showing why he is one of the best players ever. In the matches like today or against Benneteau, when he struggles, he comes up with the best shots and his best game when he needs to. That's what makes him so tough."

In the final game, Federer hit consecutive service winners, and on match point he whacked an ace at 122 mph, matching his fastest of the day. He shared a warm handshake at the net with Malisse, a friend and foe since both were juniors.

He also has a long relationship with Youzhny, who has been losing to Federer since 2000. The Russian has won three of the 32 sets they've played against each other.

"I never beat this guy," Youzhny said, "so just now I can't talk like about my dreams, what I have to do on court to beat Roger."

Like Federer, Youzhny is 30, while Malisse is 31. Two other 30-year-olds, Mardy Fish and David Ferrer, also made the round of 16 — the best such showing by 30-somethings at a Grand Slam since 1983.

"A good generation," Federer said. "Happy I'm not the only guy left, you know."

But for a while, with his back barking, Federer was feeling his age.