What to expect this time?

“Nadal always makes you run a little more, hit one more ball,” del Potro said. “And he ends up winning matches against you that you might not think he would end up winning. He knows my weak point. I can only imagine that this is going to be a super-difficult match. But if I can recover physically, and I’m feeling good, I will have my chances.”

Although del Potro’s once-vulnerable backhand is a consistently firmer shot than it was in 2017, he will still have to rely heavily on his forehand to close out points.

“Despite the improvement this year on the backhand, he can’t beat Rafa with that shot,” said Mark Petchey, a former British player who is now an analyst. “He is totally dependent on his forehand firing. Rafa, on the other hand, can beat del Potro with both his backhand and forehand.”

In last year’s Open semifinal, Petchey said, Nadal used his forehand 70 percent of the time in the opening set, which he ended up losing.

“Trying to outhit del Potro, he simply couldn’t do it,” Petchey said. “He then resorted to playing in the middle of the court and using his forehand just 53 percent of the time in the second set and didn’t have to run, and therefore closed the gaps in the court that were available in the first and also conserving huge energy while del Potro was using up his far faster.”

In theory, del Potro should be fresher. He has dropped only one set in the first five rounds, and that was against John Isner in a quarterfinal match that was played in steamy conditions but in which the rallies were rarely extended.