Spain's Rafael Nadal rolled over sixth seed Tomas Berdych 7-5 7-6 (7/4) on Saturday to set up a Cincinnati Masters final with John Isner.
Isner benefited as Juan Martin del Potro double-faulted on match point to open the door to the unseeded American, who took full advantage by rallying for a 6-7 (5/7) 7-6 (11/9) 6-3 victory.
Nadal, seeded fourth, gave himself a chance at an ATP-leading ninth title of the season and his second in a row at the Masters 1000 level after his triumph in Montreal last week.
Nadal has defeated Isner three times without a loss.
Apart from six double faults in the second set, world number three Nadal had little to complain about in victory, his 52nd match win of the season.
He is unbeaten on hardcourts, having won the title at Indian Wells in March before his Montreal win.
Nadal came back 18 hours after beating Roger Federer in their marquee quarter-final.
"I have adapted well here," said the Spaniard. "I played a great match with Roger and today I felt I also played well, even if I was a bit streaky with the serve.
"It was a challenge for me today. I never dreamed of this kind of a comeback," said the player who is now in a position to move back to second in the world behind Novak Djokovic if he wins the trophy.
"It's just unbelievable the way I have come back to be number one in the season points race. I missed the Australian Open, Miami and lost in the Wimbledon first round and still did it," Nadal said.
Nadal fired 38 winners against Berdych, who ousted Wimbledon champion and current world number two Andy Murray in the quarter-finals.
Seventh-seeded del Potro - bothered occasionally by left wrist pain this week - never found a way to get back into contention after his double-fault on match point while leading a set and 5-3.
Isner reached the second Masters level final of his career after nearly three hours of battle, turning in a 23-ace performance with 63 winners and 49 unforced errors.
He broke del Potro five times as the Argentine finished with nine double-faults.
"I made that double in the worst moment of the match. If I did it in the beginning of the match or in the third set, it wouldn't matter. But serving for the match and I made a double-fault," said del Potro, who will no doubt try to iron out the problem before the US Open begins a week from Monday.
"I'm not using that for an excuse, he played better than me in the third set," del Potro added. "He had the chance to beat me, he did very well. He made good serves, good forehands.
"I had the chance to beat him in the second set, but I couldn't close the match."
Isner was thrilled with his success.
"I can't believe I'm in the final," said the winner, who plays his best on home courts and is not enamoured of the international travel required on the ATP.
Of 10 events this season in the United States, he has reached the semi-finals or better at seven of them.
"The crowd really helped me. Ever after his match point, I knew it wasn't over until it was over. I just told myself to keep fighting," Isner said.
