NEW YORK — Normally, it takes wins in five full matches to reach a Grand Slam semifinal. Novak Djokovic has made it that far at the U.S. Open by playing only two, because three opponents pulled out of the tournament with injuries.

Djokovic, the No. 1 seed and defending champion, once again needed to put in very little work, advancing to the final four at Flushing Meadows for the 10th consecutive year when No. 9 Jo-Wilfried Tsonga stopped because of a hurt left knee Tuesday night.

The first two sets of their quarterfinal went Djokovic’s way, 6-3, 6-2. At the ensuing changeover, Tsonga was visited by a trainer and had his left leg taped below the knee. They played one point to begin the third set: Tsonga double-faulted, then retired from the match.

This came after Djokovic’s second-round opponent, Jiri Vesely, withdrew from the tournament with a sore left forearm. And then the man Djokovic was supposed to play in the third round, Mikhail Youzhny, quit after six games and only 31 minutes with a strained left hamstring.

“I put myself in a position again to be one match away from the finals. As tournament progresses, I feel like I’m getting better,” Djokovic said. “Of course, this Grand Slam is very unique for me: I never experienced something like this — to have three retirements on the road to the semifinals.”

Tsonga said when the pain came Tuesday, “I knew it was over for me straightaway.”

“It’s already tough to play against one of the best tennis players,” Tsonga said, “but when I don’t have my knee, I have no chance to come back from two sets to love.”

On Friday, Djokovic will play No. 10 Gael Monfils, who advanced earlier against No. 24 Lucas Pouille, the man who eliminated Rafael Nadal in the fourth round. Playing steadily, and with only a dose of the spectacular, Monfils reached his first major semifinal since 2008 by beating an error-prone Pouille 6-4, 6-3, 6-3 in an all-French quarterfinal.

For all of his deep runs at Flushing Meadows, Djokovic has won only two of his 12 Grand Slam titles at the hard-court tournament, in 2011 and 2015.