It is true that most fans of tennis — avid, casual and everyone between — would like to see Roger Federer meet Rafael Nadal in a U.S. Open semifinal.

But it also is true that the tennis world has a bit of a soft spot for Juan Martin del Potro, the man who on Wednesday will try to prevent Federer from advancing that far.

That already was the case before Monday, but del Potro earned even more supporters with an unlikely comeback against Dominic Thiem, overcoming illness and saving two match points to win, 1-6, 2-6, 6-1, 7-6 (1), 6-4.

The noise from the grandstand was so loud Federer heard it while playing on the show court in Arthur Ashe Stadium. “That’s the first time I experienced that,” he said later.

This time Federer will be able to hear del Potro’s fans up close as the Argentinian attempts to recreate his greatest career moment — a five-set victory over Federer in the final of the 2009 U.S. Open, after del Potro beat Nadal in the semis.

Del Potro, seeded 24th, is 5-16 against Federer, who is seeded third. But in 2012 and ’13 del Potro had a three-match winning streak in their rivalry.

“Everybody loves him; it is going to be an interesting match,” del Potro said. “It always is a pleasure to play the greatest guy in history.”

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An injury to del Potro’s left wrist mostly ruined his 2014 and ’15 seasons, but now he has a chance to make another mark at Federer’s expense.

“He was gone for so long, it’s just nice to see him back playing these kinds of matches,” Federer said. “That’s what he came back for, to get crowd support the way he got it.”

That loss in the ’09 U.S. Open cost Federer a four-Grand Slam event winning streak in 2009-10, ended his five-year U.S. Open championship streak and caused him “a lot of regrets.”

“It probably feels like one of those matches I would like to play over again,” he said. “I just had all these chances in multiple moments. The only time when he was really better, in my opinion, was the fifth set. Obviously, that was good enough to beat me that day.”

Federer said del Potro, a righthander, still has the same power in his serve and his forehand that he always had, but that his wrist problems have forced him to change his approach to backhand shots.

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“He uses his slice much more,” Federer said.

The two have met only once since 2013, with Federer winning in straight sets in Miami in March of this year.

On Wednesday, there are tennis fans who will feel strongly both ways.

Asked why he thinks fans like him so much, del Potro said: “I think people love my effort to come back and play tennis. They know what I have been through with all my wrist problems. They like one guy who never gives up, and he’s trying to play tennis.”