Reigning French Open champion Jelena Ostapenko has joined fellow top seed Kristina Mladenovic in suffering shock first-round exits at the Sydney International.
Fifth seed Mladenovic was trailing 6-4 4-2 to Australian wildcard Ellen Perez when she succumbed to fatigue during Sunday's record-breaking heatwave.
Third seed Ostapenko, who is ranked No.7 in the world, became the second big-name to fall after falling 7-6 (6-3) 6-1 to Ekaterina Makarova.
The Latvian shot to stardom last year when she became the first unseeded player in the Open era to win at Roland Garros, defeating current world No.1 Simona Halep.
However she will now need to turn around her form heading into next week's Australian Open, having also lost her tournament opener in China last week.
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Ostapenko blamed her loss to a combination of the oppressive conditions and jet-lag, having also logged trips to Thailand, Latvia and Qatar over the past few weeks.
"I'm a little bit jet-lagged. I didn't have so much time to get used to this timezone," she said.
"I just arrived from China a couple of days ago and I've been travelling a lot. I got a bit tired and all those come together. Also this hot weather today, it didn't really help me."
Earlier on Sunday, the Wollongong-born Perez defied scorching temperatures to claim her maiden WTA tour victory against world No.11 Mladenovic in just her second tournament.
The world No.343 progresses to the second round where she is likely to face Australian No.1 Ashleigh Barty, who opens her campaign against qualifier Veronica Cepede Royg on Monday.
Elena Vesnina also advanced after toppling Lara Arruabarrena in straight sets.
Fellow Australia hopeful Sam Stosur also takes to the court against lucky loser Carina Witthoeft after seventh seed Julia Goerges withdrew late with a knee injury.
In other key matches, defending women's champion Johanna Konta faces Agnieszka Radwanska and US Open titleholder Sloane Stephens battles qualifier Camila Giorgi.
Top two seeds Garbine Muguruza and Venus Williams enjoy a first-round bye.
