Australian 19-year-old Alex De Minaur saved four match points and advanced to ATP Washington Open final, where he will face third-ranked defending champion Alexander Zverev.
The Aussie teen outlasted 20-year-old Russian Andrey Rublev 5-7, 7-6 (8/6), 6-4 after two hours and 52 minutes while 21-year-old German Zverev ousted Greek teen Stefanos Tsitsipas 6-2, 6-4.
"It's one of the biggest wins of my career," De Minaur said.
"Dug deep and never gave up. Couldn't be prouder of myself."
De Minaur, ranked 72nd, won the last six points of the tie-breaker to force a third set, then broke 46th-ranked Rublev in the final game, winning on his fourth match-point chance when the Russian double faulted.
"I'm probably going to look back on that and not know how I did that," De Minaur said. "I managed to string along some great points. It was one of my best matches.
"I had to play some really good points to get myself out of there. I took advantage of a lot of short balls and tried to dictate and that turned the match around."
It will be the youngest combined age of any ATP final since 20-year-old Rafael Nadal beat 19-year-old Novak Djokovic in 2007 at Indian Wells after the ATP's first all-21-or-under semi-finals since 1995 in Buenos Aires.
"This is amazing for tennis," Zverev said. "It's good to see the young guns come up."
Zverev seeks his ninth career ATP title and third of the year after Munich and Madrid. He could join a Washington back-to-back champions list that includes Argentina's Juan Martin del Potro (2008-09) and Americans Andre Agassi (1990-91 and 1998-99) and Michael Chang (1996-97).
"I'm playing great and hopefully I can win," Zverev said.
"I hope I get out there and play a good match and get to Toronto with another title."
It's only the second career ATP final for De Minaur, the son of a Spanish mother and Uruguayan father who lost to Russian Daniil Medvedev in January's Sydney final.
Kuznetsova in final
In the companion WTA event, two-time Grand Slam champion Svetlana Kuznetsova of Russia won 6-2, 6-2 twice to reach the final, first downing Kazakh eighth seed Yulia Putintseva in a match that had been halted by rain then dispatching Germany's Andrea Petkovic.
World number 128 Kuznetsova, the 2004 US Open and 2009 French Open champion, will play for the title Sunday against the later winner between 44th-ranked Croatian Donna Vekic and China's 85th-ranked Zheng Saisai.
Kuznetsova, the 2014 Washington winner, seeks her first WTA title since Moscow in 2016 and the 33-year-old appreciated US cheers and support.
"Russia and America politically, it's probably not so good, but it's great people in sport are outside of this," Kuznetsova said.
Fognini topples Potro
los Cabos: Italian Fabio Fognini captured his first career hardcourt title on Saturday, surprising Juan Martin del Potro 6-4, 6-2, in the final of the ATP los Cabos Open.
It is the 31-year-old Fognini's eighth ATP Tour victory but his first on hardcourts after previously winning only on clay.
The win helps validate a career best season for Fognini as this is his third title of 2018 to go with wins in Sao Paulo and Bastad.
He is projected to rise one spot in the new world rankings on Monday to No. 14.
Del Potro had won their only previous meeting in 2015 in Sydney, Australia and was going for his second title in Mexico this year. He also won a tournament in Acapulco.
After dropping the first three games, Fognini fought back and used his forehand to push Argentina's Del Potro behind the baseline.
Del Potro started to come unglued and Fognini was returning everything into play as he went on to win 12 of the final 15 games of the match.