Thanks to injuries to two opponents, Novak Djokovic did not exactly need to put in much work to reach the U.S. Open's fourth round for the 10th consecutive year.

So it made sense that the defending champion and No. 1 seed would spend some extra time practicing in Arthur Ashe Stadium under the watchful eye of coach Boris Becker on Friday after spending a grand total of 31 minutes of match time on court over the second and third rounds.

"I don't think I ever had this kind of situation in my career," Djokovic said.

Yes, Djokovic got another free pass at Flushing Meadows, advancing this time when Mikhail Youzhny stopped because of a strained left hamstring while trailing 4-2 after a little more than a half-hour of play.

Youzhny received treatment from a trainer early on, getting his left leg wrapped, but briefly tried to continue. After holding serve in the sixth game, closing it with an overhead putaway up at the net, the Russian - a semifinalist in New York in 2006 and 2010, and a former top-10 player now ranked 61st - shook his head and told the chair umpire he couldn't keep going.

"Obviously, I wish Mischa a speedy recovery," Djokovic said in an on-court interview in Ashe. "He carried an injury into this match, that's what he said."

Djokovic did not play at all on Wednesday, when the man he was supposed to face in the second round, Jiri Vesely, withdrew from the tournament a couple of hours before that match because of inflammation in his left forearm.

"There's plenty of things to work on. That's the beauty of sport: Every day is different. The challenge is every day you feel different on the court. I work more or less on every aspect of my game," Djokovic said.

"The first week of a Grand Slam, obviously, things are not 100 percent, so you are getting a way through the tournament. I didn't get too much of match play," Djokovic said. "I'll try to get on the practice court once more."

And that he did, changing out of his white collared match shirt into a gray T-shirt and getting in some training. As Becker stood nearby, tossing over tennis balls, Djokovic hit serve after serve. Then he hit return after return of serves from a practice partner.

A few hundred fans remained in the stands at Ashe, enjoying that rare chance for an up-close look at a man who has won 12 Grand Slam titles, including two at the U.S. Open and two this year. Plenty of cellphone photos were taken.

In Djokovic's first-round victory way back on Monday, he experienced problems with his right arm. He was massaged by a trainer after only five games, then repeatedly shook that arm or flexed his elbow and grimaced following serves, even the ones that were far slower than he usually hits. He also dropped a set in an opening-round Grand Slam match for the first time since 2010.

Djokovic declined to discuss his health after that outing, and he hasn't really gotten a chance to test the arm in match conditions since.

On Sunday, he will be scheduled to play either 20th-seeded John Isner of the U.S. or Kyle Edmund of Britain for a spot in the quarterfinals.

After looking nearly unbeatable while becoming the first man in nearly 50 years to win four Grand Slam trophies in a row, Djokovic has had less success of late. Since completing his career Grand Slam at the French Open in June, he lost in the third round at Wimbledon, won the Toronto Masters, was upset in the first round of the Rio de Janeiro Olympics by Juan Martin del Potro, then missed the Cincinnati Masters because of a sore left wrist.