Stefanie Voegele played above her ranking as she stunned American star Sloane Stephens 6-4 5-7 6-2 in Acapulco on Thursday.
US Open champion Sloane Stephens advanced in Acapulco thanks to Wednesday's 5-7 7-6 (8-6) 6-0 victory over Dutch qualifier Arantxa Rus.
US Open champion Sloane Stephens secured her first win since her grand slam success last year.
American women struggled on the first day of the Australian Open, with both Venus Williams and Sloane Stephens falling at the first hurdle.
American 13th seed and US Open titleholder Sloane Stephens, 24, was upset 2-6 7-6 (7-2) 6-2 by Zhang Shuai on Monday.
Pam Shriver feels Sloane Stephens has had "a big lie down" after her US Open triumph but will get back on her feet before long.
Sloane Stephens failed to stop the rot at the Sydney International, Camila Giorgi thrashing the US Open champion in the first round.
The Fed Cup final went to a deciding doubles rubber, where the United States got the better of Belarus.
CoCo Vandeweghe won the first point of the Fed Cup final, but 19-year-old Aryna Sabalenka made it 1-1 by beating the US Open champion.
The draw for the Fed Cup final was a grand affair, but Coco Vandeweghe may have been feeling a little nervous ahead of the tie with Belarus.