Rafael Nadal advanced through to the second round of the 2018 U.S. Open on Monday night after David Ferrer retired in the second set at Arthur Ashe Stadium.

Nadal won the first set 6-3 but trailed 3-4 in the second before a persistent leg injury forced Ferrer to concede the match.

Trailing 1-2 in the second set, Ferrer had the trainer look at his left leg. The trainer worked on the leg for an extended period of time before Ferrer returned to action.

The two-time U.S. Open semifinalist still managed to win the next two games before he once again called over the trainer, who began focusing on Ferrer's calf and Achilles. While Ferrer continued to play, it looked likely he'd be able to see out the full match:

Nadal was far from his best on the night. He committed 22 unforced errors with 16 winners through the two sets. His return game was on point, though, as he earned six break-point opportunities in just eight service games for Ferrer:

If he had been healthy, Ferrer may have pushed Nadal to the brink. The top seed in the women's draw, Simona Halep, is already out of the U.S. Open after the first round, and Nadal could've been in similar danger.

However, the injury made an upset all but impossible for Ferrer.

Monday's match is a disappointing end to the 36-year-old's Grand Slam career. He told Sportsnet's Arash Madani earlier this month he'd bow out of Grand Slam events after the 2018 U.S. Open and formally retire at some point during the 2019 season.

The U.S. Open offered a goodbye:

Nadal, meanwhile, will shift his focus to Vasek Pospisil in the second round. Pospisil beat Lukas Lacko to get his tournament started. The two have met just once before, with Nadal winning in straight sets at the 2015 China Open.

After he withdrew from the Western & Southern Open, it's not entirely surprising Nadal lacked his usual sharpness Monday night. Ferrer was a game challenger as well.

The defending U.S. Open champion should start rounding into form when he steps on the court against Pospisil.