Spanish teenager Carlos Alcaraz held his nerve in the final-set tie-break to beat Matteo Berrettini 6-1 6-7 (2/7), 7-6 (7/5) in the quarter-finals of the ATP event in Vienna on Friday.
He will next face Alexander Zverev, the German second seed who beat Felix Auger-Aliassime 6-4, 3-6, 6-3 to avenge a fourth-round loss to the Canadian at Wimbledon this year.
In the day's third quarter-final, American qualifier Frances Tiafoe, who knocked out top seed Stefanos Tsitsipas on Thursday, continued his run with a 6-4, 7-6 (8/6) victory over Argentine eighth seed Diego Schwartzmann.
The 18-year-old Alcaraz easily took the first set, lost the second on a tie-break before sealing victory over the Wimbledon finalist on his third match point in the third-set tie-break.
Alcaraz broke the Italian's first two service games of the match and cruised to the first set in 29 minutes.
The second set went with serve, although Berrettini saved two break points in the fifth game before taking the tie-break comfortably.
Alcaraz broke the Italian in the fourth game of the final set and led 4-1 but Berrettini broke back to love in the seventh game.
After Alcaraz raced to a 6-3 lead in the tie break, Berrettini held his serve on the first two before the Spaniard sealed victory on his own serve to reach his first ATP 500 semi-final.
"This is one of my best performances on tour so far," Alcaraz said.
"I am really happy for that and it was amazing. I just tried to play aggressively all the time. In the tough moments it was important to be able to do that."
He will face Zverev whose match against Auger-Aliassime contained a break in each set.
The German broke in the third game of the first set and the sixth game of the decider and was dominant on his own serve except for the seventh game of the second set when the Canadian broke to 30.
"I did make it difficult for myself once again, that needs to stop happening," Zverev said. "One easy mistake and the second set went away."
Alcaraz is the fifth-youngest semi-finalist in an ATP 500 event. The record is held by Zverev, who reached the last four in Hamburg in 2014 aged 17.
"He has improved a lot," said Zverev. "He has made big steps forward and is a great player, so I am looking forward to that match."
The Spaniard reached 38 in the world in September after progressing to the last eight at the US Open -- the youngest men's quarter-finalist in that Grand Slam tournament in 58 years.
At 18 years and four months, he was the youngest player to be ranked so high since compatriot Rafael Nadal, who was 17 when he reached No. 37 in April 2004.