Back injury forces Stan Wawrinka to withdraw from quarter-final against fellow-Swiss Roger Federer.
Greek third seed Stefanos Tsitsipas battled back from a set down to beat Serbia's Filip Krajinovic 3-6, 6-4, 6-4 in the quarter-finals of the Swiss Indoor championship on Friday.
The win took Tsitsipas into his 10th semi-final of the season where he will face top seed Roger Federer, who was handed a walkover after Swiss compatriot Stan Wawrinka withdrew before their quarter-final with a back issue.
Federer is looking to win his 10th title in Basel and his third in a row for a second time after a hat-trick of titles from 2006-2008.
Tsitsipas fired 12 aces and 38 winners in a match where both players dropped serve three times, but the Greek 21-year-old was far more effective on his second serve.
It was the 11th time this season that he had come back from a set down to win a match.
The other semi-final is between unseeded pair Alex de Minaur of Australia and American Reilly Opelka.
Wildcard De Minaur won 6-4, 7-6(4) against Germany's Jan-Lennard Struff while big-serving Opelka sent down 31 aces as he beat Spanish fourth seed Roberto Bautista Agut 6-3, 3-6, 6-3.
Berrettini sets up Thiem semi-final in Vienna
Italy's Matteo Berrettini had to dig deep to beat Russia's Andrey Rublev 7-5, 7-6(4) at the Vienna Open on Friday and set up a semi-final clash with top seed Dominic Thiem.
The win over the Kremlin Cup champion saw third seed Berrettini reach his he Italian, who is currently eighth in the Race to London rankings behind German Alexander Zverev, will qualify if he beats Thiem.
"I am really happy. It was a tough match, mentally and physically," Berrettini said.
"Andrey is playing his best tennis so far. Last week, he won Moscow, so he was eighth semi this year and improved his chances of qualifying for next month's season-ending ATP Finals.
Six spots have already been filled and tfeeling confident and I saw it on the court."
Berrettini won 80% of his first serve points and converted a break point in the opening set -- the only break in the match -- to eliminate the 22-year-old.
Earlier, Spaniard Pablo Carreno Busta was forced to retire from his quarter-final match with Thiem leading 5-0 in the opening set, allowing the Austrian to move into the last four.
Second seed Karen Khachanov was beaten 7-6(6), 6-2 by Argentine Diego Schwartzman, who was playing his 60th match of the year, for his second win in three matches over the Russian.
Khachanov served for the opening set at 5-4 but Schwartzman levelled and then went on to win a close tiebreak when the Russian double-faulted on set point.
Schwartzman broke in the opening game of the second set and again in the fifth before closing out the contest on his first match point.
