NEW YORK • As Novak Djokovic pursues a third US Open title, he finds himself in the odd position of issuing get-well messages to a chorus line of incapacitated opponents.

Tennis is not a contact sport, but for many of those across the net from Djokovic, these have been a treacherous few days.

"I can only wish all of my opponents a speedy recovery," Djokovic said after another injury struck down his latest opponent.

It has been the oddest of tournaments for world No. 1 Djokovic.

First, Jiri Vesely, exhausted by a five-set match in the first round, pulled out before a second-round match with Djokovic. Then, in the third round, Mikhail Youzhny had to retire with a leg injury during the first set, which Djokovic led 4-2.

Kyle Edmund actually remained upright for a three-set match against Djokovic in the fourth round, but then it was back to the infirmary for Jo-Wilfried Tsonga in the quarter-finals on Tuesday.

Djokovic took the first two sets, 6-3, 6-2, before Tsonga received treatment for his left knee and had to retire.

"It's something I had already in the past," Tsonga said.

"When I have my knee, of course, it's already tough to play against one of the best tennis players. But when I don't have my knee, I have no chance to come back from two sets to love."

Djokovic has played only six sets in his last four scheduled matches.

"This Grand Slam is very unique for me," he said.

"I never experienced something like this, to have three retirements on the road to the semi-finals."

At the end of a long season, Djokovic said, the rest will serve him well, especially because he entered the tournament with a sore left wrist.

Now he will face an old adversary of sorts in Gael Monfils, who beat fellow Frenchman Lucas Pouille 6-4, 6-3, 6-3 to gain a spot in a US Open semi-final for the first time in his career.

Monfils has entertained tennis fans for more than a decade with his balletic athleticism and his penchant for the occasional trick shot.

He might hit a ball through his legs for no compelling reason, jump off the back wall to retrieve a ball, or dive while reaching for a drop shot.

Djokovic said Monfils was one of the few players he would pay to watch, because of his theatrical style of play and his cheerful approach.

But in 122 minutes on court on Tuesday, Monfils displayed virtually none of those attributes, other than one body-bending slide early in the match.

Instead, the Frenchman remained on his feet, and a vast majority of his shots were the stuff of a technical tennis manual rather than a YouTube highlights reel.

"He seems more focused at this time of his career," Djokovic said.

Monfils' chosen style of play proved the right one against an exhausted Pouille, who had earlier upset Rafael Nadal in a stout-hearted, dramatic five-setter. But the stakes will rise significantly now.

Djokovic has beaten Monfils in 12 of their 13 encounters: Monfils won a match in a Futures tournament - a kind of minor league to the main Tour - in 2004, and Djokovic won their next 12 matches, beginning at the 2005 US Open.

"He's a better player than me, definitely," Monfils said.

"I think I have no shame to say it. He is better than me.

"But I'm winning more, you know, more matches. So it's tougher for some people to say that I'm just a showman."

NEW YORK TIMES, REUTERS