Venus Williams lost in straight sets and Victoria Azarenka retired part-way through her match on another day of drama at the Mubadala Silicon Valley Classic.
Number three seed Williams - the highest seed left in the draw - was stunned by world number 49 Maria Sakkari, losing 6-4 7-6 (7/2) in the quarter-finals.
The Greek player took just an hour and 43 minutes to emerge victorious, reeling off five straight games after going down 3-0 in the opening set.
The second set saw the players exchange four breaks of serve each, before Sakkari came out stronger in the tiebreak.
She will now face Danielle Collins in the final four, after the American's opponent, Azarenka,retired with a back injury in their quarter-final match.
Collins had been 3-0 up in the second set - having lost the first on a tiebreak - when Azarenka put an end to the match.
The second semi-final will feature Elise Mertens, who beat Britain's Johanna Konta 7-6 (7/4) 6-3, and number five seed Mihaela Buzarnescu, who defeated Ajla Tomljanovic 6-1 7-5.
Meanwhile, rain affected the day's play at the Citi Open in Washington, with only one of the quarter-final matches completed.
Germany's Andrea Petkovic produced an upset by defeating number six seed Belinda Bencic 6-3 2-6 7-6 (8/6).
The 2013 runner up had been 4-2 down in the final set before rallying to force a tiebreak - and then saved three match points before clinching the win.
Svetlana Kuznetsova was one set up against Yulia Putintseva, while Donna Vekic was 3-0 up in the first set against Magda Linette, before the weather stopped play.