By Larry Fine

NEW YORK (Reuters) - Top seed Novak Djokovic cannot seem able to get a match in at the U.S. Open, as the world number one was given another rapid advance when Russian Mikhail Youzhny retired due to a leg injury when trailing 4-2 on Friday.

Youzhny was treated during the changeover at 4-1 in the third-round match at Arthur Ashe Stadium and had his left thigh taped. He won the next game on his serve but could not continue, giving Djokovic a pass into the round of 16.

The Serb, winner of this year's Australian Open and French Open to complete a career grand slam, came into the match off a three-day break due to a walkover into the third round when Jiri Vesely of the Czech Republic withdrew.

"It's difficult to pick the words now. I don't think I ever had this kind of situation in my career, where I had a second-round walkover and the third round 30 minutes only," Djokovic said on court before heading off to practice.

"I'll try to get the positives out of these six games. Obviously I did start very well with a good intensity and I'll try to carry that into the next match."

Defending champion Djokovic will next face either 20th-seeded American John Isner or 84th-ranked Briton Kyle Edmund with a berth in the quarter-finals at stake.

Djokovic, who has been bothered in recent months by a nagging wrist injury, won his opening match at Flushing Meadows on Monday, 6-3 5-7 6-2 6-1 over 247th-ranked Jerzy Janowicz of Poland.

(Editing by Mark Lamport-Stokes)