Third seed Stefanos Tsitsipas beat Filip Krajinovic 3-6 6-4 6-4 to book a semi-final with nine-time champion Roger Federer at the Swiss Indoors.
The 21-year-old world number seven served 12 aces and beat the 46th-ranked Serbian in an hour and 53 minutes.
Fourth seed Roberto Bautista Agut's hopes of reaching next month's ATP finals were dented by a 6-3 3-6 6-3 defeat against American Reilly Opelka.
World number 37 Opelka served 31 aces in his win in an hour and 33 minutes.
With the top eight players qualifying for the O2 Arena from 10-17 November, ninth-placed Bautista-Agut was trying to keep pace with eighth-ranked Matteo Berrettini, but the Italian reached the semi-finals of the Erste Bank Open in Vienna to increase his lead to 130 points.