Juan Martin del Potro double-faulted on match point to open the door for American John Isner, who turned the tide and triumphed in three sets in their Cincinnati semi-final on Saturday.
Isner defeated the seventh-seeded Argentine 6-7 (5/7), 7-6 (11/9), 6-3 to book a place in the Masters final, where he'll face either fourth-seeded Rafael Nadal or Czech Tomas Berdych.
Del Potro, who had been bothered by nagging left wrist pain this week, never found a way to get back into contention after his gift to the unseeded American, whom he had beaten in all four of their previous meetings including the Washington final earlier this month.
Isner reached the final 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.
Del Potro ended with nine double-faults and will surely find time to work on his serve with the US Open starting a week from Monday.
"I made that double in the worst moment of the match. If I did it the beginning of the match or in the third set, it wouldn't matter. But serving for the match and I made double-fault.
"Today I was like a very young player. He took his chances and came back in the match," del Potro said.
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.
"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."
Del Potro, winner of two titles this season, did not list trouble as the reason for the defeat.
"I'm not using that for an excuse, he played better than me in the third set. 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."
After del Potro's costly double, Isner began to slowly find his range, claiming the second set after a lengthy 71 minutes as del Potro saved four set points before hitting the net on a fifth.
Isner sprinted ahead with a break in the second game of the third set and gave himself two match points with an ace.
One was enough as he advanced into the title match.
It was his fourth semi-final at the elite Masters 1000 level and gave him the chance at an eighth ATP career title.