NEW YORK — In the 25th anniversary year of his memorable run to U.S. Open semifinals at age 39, Jimmy Connors came up a lot Monday.

There was Connors’ name in the same sentence as that of Juan Martín del Potro, who at No. 142 was trying to become the lowest-ranked U.S. Open quarterfinalist since 174th-ranked Connors in 1991.

There was Connors’ name in the same sentence as that of Ivo Karlovic, who at 37 was the oldest player in the fourth round since Connors in ’91.

Only one of them, del Potro, will continue to be compared to Connors this week.

Karlovic lost to sixth-seeded Kei Nishikori, 6-3, 6-4, 7-6 (4), while Del Potro, 27, advanced to his first Grand Slam quarterfinal in three years when eighth-seeded Dominic Thiem retired from their fourth-round match with a right knee injury. Del Potro led, 6-3, 3-2, at the time.

Del Potro has his own history of injuries, having returned in February from a third operation on his right wrist.

Del Potro and Gael Monfils are the only players in men’s singles who have not lost a set in the tournament. Del Potro, who has said he is tired from his run to winning the silver medal at the Rio Olympics last month, may ultimately benefit from shorter matches. He will play third-seeded Stan Wawrinka in the quarterfinals, with a potential meeting against No. 2 Andy Murray in the semifinals.

Del Potro upset Wawrinka in the second round at Wimbledon, a harbinger of good things this summer. At the time, del Potro said he was just happy to be playing at Centre Court, happy to be playing tennis at all.

That was his first Grand Slam tournament since his latest wrist operation, before which he seriously considered retiring. He said he did not expect to reach the third round. He acknowledged Monday that his quarterfinal appearance here was a surprise, too.

Del Potro considers Wawrinka the favorite in their rematch, but that “anything can happen in this event for me.”

“I got the power from the crowd in every match,” he said, “and that’s helped me a lot to fight and to play my best tennis.”