In temperatures reaching 40 degrees Celsius (104 Fahrenheit) for much of the match, Wozniacki lost 7-5, 6-7 (6), 6-4 to Barbora Strycova in a duel that stretched to 3 hours, 19 minutes.
Both players were treated for foot injuries during a grueling second set on Ken Rosewall Arena. Wozniacki came back from 5-2 down in that set, and then 5-0 in the tiebreaker, to win it and force a third set.
”It was brutal out there … but you just try and think like you’re on a beach drinking pina coladas,” Wozniacki said. ”That’s basically your train of thought. You know that it’s the same for both players, so I was just trying to mentally just try and keep cool.”
Strycova will meet the winner of the night match between second-seeded Agnieszka Radwanska and qualifier Duan Yingying.
Former Wimbledon finalist Eugenie Bouchard continued her strong start to the year with a 6-2, 6-3 quarterfinal win over Anastasia Pavlyuchenkova.
Bouchard had a breakout season in 2014, reaching the semifinals at the Australian Open and French Open before making the final at Wimbledon. But she has only gone past the fourth round at a major once since then and slipped to No. 46 at the end of last year.
The Canadian’s semifinal opponent will be Sydney-born Johanna Konta of Britain, who beat Daria Kasatkina 6-3, 7-5. Konta broke Kasatkina’s service in the 11th game, helped by a double-fault to set up break point in that game.
Last year at the Australian Open, Konta became the first British women to make a Grand Slam semi since 1983 before losing to eventual champion Angelique Kerber.
At Auckland, New Zealand, American John Isner narrowly avoided the fate of two former champions when he beat Tunisia’s Malek Jaziri in a third-set tiebreaker to advance 6-3, 3-6, 7-6 (6) to make the quarterfinals of the ASB Classic.
Isner won his last three points with volleys at the net.
”The way I need to finish points is that if I can I have to try and finish them at the net,” Isner said. ”I did that three times in a row and I’m very proud of that.”
Four former champions were scheduled to play second-round matches Wednesday but by the time the second-seeded Isner took the court, two had bowed out.
Defending champion and No. 1-seeded Roberto Bautista Agut was forced to withdraw before play began because of a stomach virus, handing New Zealander Ruben Statham a place in the second round as a lucky loser from the qualifying rounds.
The 2015 champion Jiri Vesely was due to play Bautista Agut and thought he had caught a break when the Spaniard withdrew and the 434th-ranked Statham took his place. But Vesely had a tussle on his hands before winning 6-7 (6), 7-5, 6-3.
Four-time champion David Ferrer was first up on center court and lost 6-2, 4-6, 7-6 (4) to Robin Haase of the Netherlands.
At the WTA’s Hobart International, former French Open finalist Lucie Safarova was beaten 2-6, 6-3, 7-5 by Japanese qualifier Risa Ozaki.
Ozaki will next meet Romanian Monica Niculescu.
Top-seeded Kiki Bertens advanced to the quarterfinals with a 6-1, 6-4 win over Galina Voskoboeva of Kazakhstan.