Second seed Garbine Muguruza cooly saw off Japanese qualifier Mai Hontama 6-3, 6-2 to reach the semi-finals of the Chicago Classic on Friday.
Muguruza will face Marketa Vondrousova after the Czech collected a 6-7 (4), 6-0, 7-6 (3) win over 10th-seeded American Danielle Collins.
Elena Rybakina advanced to the semis in Chicago against Belinda Bencic after the Swiss was forced to retire with a right knee injury, trailing 6-4, 3-1.
“I think what happened with Belinda was not even about today’s match as much as jet lag and the combination of everything,” Rybakina said.
“Many players have just arrived here from Europe, so we were in America, then Europe for a few weeks. It’s not easy at all.”