It was late Tuesday. The postmatch news conference was over, and Novak Djokovic was running toward the locker room at Arthur Ashe Stadium with energy still to burn after another unexpectedly short shift at the United States Open.

It has been that sort of Grand Slam tournament, a truly strange trip to the semifinals in which his opponents have vanished like mirages when they have come close enough to play. They retired midmatch with injuries, in the cases of Mikhail Youzhny, in the third round, and Jo-Wilfried Tsonga, in the quarterfinals. Or they were unable to play at all, in the case of Jiri Vesely, in the second round.

“Novak’s got a good witch doctor,” said Thierry Ascione, Tsonga’s coach.

Perhaps it would be prudent for Djokovic’s next opponent, the flashy and often-injured Frenchman Gaël Monfils, to encase himself in Bubble Wrap before their semifinal match on Friday.

“Look, I never had a Grand Slam where I had three retirements up to the semifinals, but I’ve got to take it, I guess,” Djokovic said Tuesday night after Tsonga, having lost the first two sets by 6-3, 6-2, retired with a knee injury.