No.8 seed CiCi Bellis scored a statement win over two-time Wimbledon champion Petra Kvitova, needing just over one hour to book her spot into the Bank of the West Classic semifinals.
Bellis, ranked 44th in the world, yesterday was to take on fellow Californian Vandeweghe in a semi-final, while Muguruza is to face third-seed Keys.
The crowd came alive behind the Atherton-raised Bellis when she broke serve to trail 5-3 in the first set, lacing a pure backhand victor to close out the game.
Although she was broken early in each set, Keys showed plenty of resolve in breaking back both times.
She returned in May at the French Open, losing in the second round. Since the start of this year's grass-court season, she has been coached by Pat Cash, the 1987 Wimbledon champion known for his aggressive style.
Bellis will square off against sixth-seeded American CoCo Vandeweghe in the semis.
"This whole tournament for me has made me feel so great".
It was the initial career meeting between the two young Americans and Vandeweghe used her experience to dominate the 65-minute contest.
"I'm five years older, and hopefully I'm more mature", Vandeweghe said.
"I think today I was very accurate. You grow and evolve as a tennis player and as a person, and I think especially in the last two years, it's been kind of clicking in that regard". After a tournament win in Birmingham, she fell in the second-round at Wimbledon as well, where she had to call for medical assistance in her three-set loss to Madison Brengle.
Kvitova dropped the first game to love on serve and it did not get much better from there as she ran up 26 unforced errors. She also controlled the net, winning 10 of 13 points there.