Donna Vekić gained a surprise win over Sloane Stephens as the first round of the Pan Pacific Open in Tokyo came to an end.
The Croatian beat the American world number nine 6-4, 6-4 to set up a second round match with Briton Johanna Konta, to be played Thursday (September 20).
"I felt good," Vekic said, after her win against the 2017 US Open champion at the Arena Tachikawa Tachihi.
"I beat her last time we played, so I knew my game plan and I tried to stick to it.
"I'm playing really well this year.
"I'm improving a lot.
"Even when I lose, I keep working hard and doing the right things, and I feel like it's showing on the court."
Konta had emphatically defeated Gabriela Dabrowski of Canada 6-2, 6-0 earlier in the day.
In other matches, Alison Riske of the United States got past Canadian Eugenie Bouchard, Victoria Azarenka of Belarus beat local wildcard Kurumi Nara and Czech Babora Strýcová secured a victory over Zarina Diyas of Kazakhstan.
American Coco Vandeweghe took her doubles partner Ashleigh Barty to a third set before the Australian got the better of her 6-4, 3-6, 5-7.
The tournament, a Women's Tennis Association Premier event, continues tomorrow.