The Belgian has already collected a pair of grand slam doubles titles this year at the Australian Open and Wimbledon and is seeking her seventh career singles crown.
She will need to get past No.4 seed Daria Kasatkina to have a chance.
The Russian outlasted Magda Linette 6-4 3-6 6-4 in her quarter-final and is seeking her third title of 2021 after a three-year drought between wins.
The other semi-final will match No.7 seed Danielle Collins against qualifier Ana Konjuh.
Collins pulled out a tight 7-6 (7-5) 7-6 (7-4) upset of No.2 seed Elena Rybakina to reach the semis in San Jose for the second time.
The American won her eighth consecutive match after taking the title at Palermo two weeks ago.
She will meet the only unseeded player remaining, Konjuh, who won the final 10 games of the match to close out Zhang Shuai 3-6 6-2 6-0.