By Larry Fine

NEW YORK (Reuters) - Top seed Novak Djokovic advanced to the semi-finals of the U.S. Open when his Serbian Davis Cup team mate Janko Tipsarevic retired in the fourth set of their quarter-final on Thursday.

World number one Djokovic was leading 7-6 6-7 6-0 3-0 when Tipsarevic, who had his leg strapped during the match, quit after three hours, 27 minutes of play at Arthur Ashe Stadium.

"Congratulations to Janko for a fantastic tournament and a great effort today," Djokovic told the center court audience. "He's a great friend of mine and it's not easy to play against him today."

Djokovic said he realized his compatriot was in trouble after the first two hotly-contested tiebreaker sets. The top seed won the first tie-break 7-2, and lost the second 7-3.

"I sensed when I made a break in the third set that he lost a lot of pace on his first serve and was not able to move so well," the Serb added.

Djokovic also gave onlookers a scare when he doubled over and called for the trainer after making a spectacular sliding forehand winner in the first game of the fourth set.

After getting the big toe on his left foot treated and bandaged, Djokovic went back to business before the 20th-seeded Tipsarevic signaled he had had enough.

"My left toe is bleeding," explained Australian Open and Wimbledon champion Djokovic. "These things happen all the time when you're sliding like I do."

Djokovic will meet the winner of Thursday's night match between five-time Open winner Roger Federer and France's Jo-Wilfried Tsonga.

(Editing by Mark Lamport-Stokes)