Sloane Stephens's Australian Open buildup suffered a blow after the former US Open champion lost 6-4 3-6 7-6(5) to China's Yuan Yue in the Hobart International first round on Tuesday.
The duo slugged it out for nearly three hours before the Chinese world number 73 secured a memorable victory.
Having split the first two sets, Yuan broke Stephens when the American was serving for the match at 6-5. Stephens led 5-4 in the tiebreak but could not hold on to her advantage.
"I'm very happy and excited to win," a beaming Yuan said.
"She's a Grand Slam champion. At home, I'd (often) watch her play on the TV. I'm happy to be here (playing against her)," she said of her opponent.
Former Australian Open champion Sofia Kenin began her campaign at the Melbourne Park warm-up event with a 6-3 6-3 win over Greet Minnen of Belgium.
Kenin, who saved all three break points she faced, will meet Daria Saville for a spot in the quarter-finals.
Britain's Emma Raducanu withdrew from a charity match in Melbourne on Tuesday, which was part of her Australian Open buildup, the BBC reported.
Her original opponent Naomi Osaka had pulled out earlier and was replaced by Donna Vekic for the contest, which was cancelled.
In the first round of the Adelaide International, sixth seed Jelena Ostapenko came back from a sluggish start to beat Sorana Cirstea 2-6 6-2 6-4.
Ostapenko needed nearly two hours to secure victory at the Australian Open tune-up event, enjoying a modicum of revenge after the Romanian beat her at Wimbledon last year.
Cirstea won four games in a row to race through the opening set, but former French Open champion Ostapenko hit back and fired back-to-back aces to force the decider.
The Latvian raced into a 5-2 lead but was broken serving for the match. She blew two match points at 5-3 before sealing victory with her sixth ace.
She next meets Caroline Garcia, who rallied past 18-year-old Australian wildcard Taylah Preston 6-4 1-6 6-3.
World number seven Marketa Vondrousova withdrew from the event ahead of her first-round match due to a hip injury.
Laura Siegemund, part of the Germany team that won the United Cup title last week, upset world number 14 Liudmila Samsonova 6-7(1) 6-4 6-4.
Elsewhere, Arthur Fils ended Richard Gasquet's title defense at the Auckland Classic with a 6-3 6-4 win over his French compatriot.
"He's a great champion," Fils said of former world number seven Gasquet.
"He won here last year, so I was a little bit nervous before the match, but I´m really happy to play against him."
Returning to the tour for the first time since last year's Wimbledon, Denis Shapovalov lost 6-4 6-2 to Sebastian Ofner.
