Maria Sakkari sprung a surprise on Friday to end Venus Williams' hopes of reaching a ninth final at the Silicon Valley Classic.
Williams won this event in 2000 and 2002, earning her eighth appearance in the decider as recently as 2016.
But the 38-year-old third seed came unstuck in the quarter-finals, Greek world number 49 Sakkari winning 6-4 7-6 (7-2) in the WTA Premier event in San Jose.
A chaotic second set produced eight breaks of serve, but Sakkari held throughout the tie-break to secure passage to the last four.
Barring her path to the decider will be another American in Danielle Collins, who benefitted from the retirement of Victoria Azarenka during their match to advance.
Former world number one Azarenka, a two-time Australian Open winner, had won the first set 7-6 (7-4) but was trailing 0-3 in the second when she withdrew.
Serena Williams' conqueror Johanna Konta saw her run in the tournament come to an end, fourth seed Elise Mertens winning their contest 7-6 (7-4) 6-3.
The Belgian will take on Mihaela Buzarnescu in the last four, the Romanian fifth seed having overcome Ajla Tomljanovic 6-1 7-5.
In the only match that was able to be completed on a day of bad weather at the Citi Open in Washington, Andrea Petkovic upset Belinda Bencic 6-3 2-6 7-6 (10-8).
Svetlana Kuznetsova had taken the first set against Yulia Putintseva and Donna Vekic was leading Magda Linette 3-0 in the opening set of their encounter when rain interrupted play.
The match between Zheng Saisai and Allie Kiick was yet to start when organisers called a halt to the day's play, with action set to start early on Saturday in attempt to make up for lost time.