Anderson, 31, beat the 12th-seeded Pablo Carreño Busta, 4-6, 7-5, 6-3, 3-4, in the first semifinal of the day, which had a distinct warm-up act feel to it.
The marquee event came later in an intriguing matchup between del Potro, the 2009 champion still working his way back from multiple wrist surgeries, and the 31-year-old Nadal, a winner here in 2010 and 2013 who is starting to look as good now as he did then.
“I was playing so-so at the beginning of the tournament,” Nadal said, “and I have been playing better and better every day.”
That does not bode particularly well for Anderson. The oft-injured South African possesses a powerful serve that could trouble Nadal on Sunday in their fifth head-to-head encounter. Nadal won their four previous meetings and has 15 major titles, second only to Federer’s 19. He also earned his 15th consecutive Grand Slam semifinal victory, a streak that began after del Potro beat him here in 2009.
Nadal has been rejuvenated for much of the year, reaching the Australian Open final (he lost to Federer) and soaring during the clay season to notch his record 10th French Open title. He has not been perfect during his U.S. Open run, but he is getting stronger, combining a relentless defense with an increasingly sharp serve and the shrewd tactics of a veteran clay-court specialist.
“He played so smart from the second set till the end of the match,” del Potro said of Nadal. “I feel better with my backhand, but it’s not good enough to win a title like this.”
After a first set in which Nadal seemed reluctant to hit the ball to the Argentine’s high-powered forehand, Nadal used his strategic acumen to change the course of the match.
He had been peppering del Potro’s weaker backhand cross court with his lefty forehand, but del Potro was equal to the challenge, at least in the first set. He camped out on the ad side of the court and waited for the right moment to hit penetrating backhands down the line for winners.
Nadal was frustrated, but during the changeover he switched to Plan B. No matter how frightening del Potro’s forehand might be, Nadal would have to hit to that side, if only to erase his predictability and force del Potro to move. Now, no longer able to stand in the corner and set his feet, Del Potro struggled. He had also just come off two difficult matches, as well as an illness, and Nadal made him pay for all of it.
As del Potro wore down, he had increasing difficulty with his forehand and, even more, with his backhand, the shot most affected by his wrist injuries.
“Most important thing is, after the first set, in my opinion, he didn’t hit balls in a row from good positions,” Nadal said. “That makes the difference.”
During one stretch, Nadal won nine consecutive games, and 10 out of 11 in the second and third sets. While del Potro staggered, Nadal only gained strength, urging himself on with repeated fist pumps. With one more win, he will match Federer’s two improbable major titles in 2017.
“It is probably the most important match for me that remains this year,” Nadal said.
While Nadal consolidated his hold on the No. 1 ranking, Anderson, at No. 32, is the lowest-ranked player to reach a U.S. Open final since the inception of the rankings system in 1973.
Anderson and Carreño Busta don’t have the name recognition of Nadal or even of del Potro, so the audience at Arthur Ashe Stadium was relatively quiet and the mood somewhat subdued.
But it did not matter to the emotional Anderson, who is also the lowest-ranked finalist at a major tournament since Jo-Wilfried Tsonga reached the final of Australian Open in 2008 at No. 38.
He was already the first South African to reach a U.S. Open men’s semifinal since Johan Kriek in 1980 (Kriek lost to Bjorn Borg, who then lost to John McEnroe). Cliff Drysdale was the last player from South Africa to reach the final at the U.S. championships, in 1965. He lost to Manuel Santana of Spain.
After Anderson beat Carreño Busta, he raised his arms, took off his white cap and climbed into his team box to celebrate in a fashion usually reserved for the championship round. Unusual, yes, but against Nadal he may not have another chance.
“I will be playing for a Grand Slam trophy,” Anderson said. “That’s an amazing feeling. I have to get ready. Still obviously a very difficult match ahead of me, but I think right now I’m just trying to unwind a little bit and just enjoy sort of today’s match.”
An earlier version of this article misidentified the last South African man to reach a Grand Slam final before Kevin Anderson. He was Kevin Curren at the 1984 Australian Open, not Johan Kriek at the 1981 Australian Open.