Top-ranked Daniil Medvedev committed 11 double-faults in falling to Stefanos Tsitsipas 7-6 (8/6), 3-6, 6-3 in Sunday's semi-finals of the ATP and WTA Cincinnati Masters.
Medvedev helped seal his own fate in the sixth game of the final set as he double-faulted four times to hand his Greek opponent to a 4-2 lead.
Tsitsipas will play for the title on Sunday against Croatian Borna Coric, who returned to an ATP Masters final for the first time in four years by defeating Britain's Cameron Norrie 6-3, 6-4. Tsitsipas won only his third match against Medvedev after losing seven and also defeated a reigning number one player for only the second time in 12 career attempts.
The fourth seed put a winner into the corner on the first of three match points to earn the final.
"I knew I would have a difficult task in the third set," Tsitsipas said. "He made it a very physical match, very demanding.
"But I took advantage of his missed first serves. They gave me time to think of my next move clearly. I was very calm and concentrated on every single task."
World number 152 Coric, who missed a year of play with shoulder problems and only returned to the tour in March, advanced in 91 minutes in a match delayed for hours by an afternoon of rain.
"It was a long day. I didn't expect to be playing in the evening," Coric said. "It was a crazy day but it finished very well."
Coric is the second-lowest-rated finalist since the rankings began in 1973, with number 191 Andrei Pavel setting that standard as he played for the Roland Garros title in 2003.
Coric got past a slow start marred by 14 unforced errors in the first six games, settling in to lift the level of his tennis.
The Croat, who lost a Masters final at Shanghai in 2018 to Novak Djokovic, limited Norrie to nine winners while sending over 17 along with 24 unforced errors.
"I played extremely well, but in the beginning I was not there," Coric said. "I then found my rhythm and began serving better. That was the key to the match."