Top-seeded Garbine Muguruza fell to the court behind the baseline in the third set before retiring from her opening match at the Brisbane International because of leg cramps. The Wimbledon champion was up a set and a break on Tuesday but couldn't close it out in straight sets as Aleksandra Krunic rallied from 5-2 down in the second, winning it in a tiebreaker to force the match into a third set.
Muguruza, who held the No. 1 ranking for four weeks last year, lasted three games in the third before being stopped by the cramps after almost 2 ½ hours in humid conditions on Pat Rafter Arena, giving Krunic a 5-7, 7-6 (3), 1-2 retired, victory.
"I don't really cramp a lot, so I'm actually surprised that today my body was like that," Muguruza said, adding that her calf muscles started to cramp early in the second set. "It's a shame because I always come here excited about the first tournament, and this one was bad luck, I guess."
Krunic advanced to a quarterfinal match against seventh-seeded Anastasija Sevastova, who beat Sorana Cirstea 6-2, 6-1.
Alize Cornet reached the quarterfinals with a 6-1, 7-5 win over Mirjana Lucic-Baroni.
The Brisbane International had been Muguruza's only scheduled tuneup event before the Australian Open, the first major of the season starting Jan. 15.
"I'm going to evaluate what options do I have now ... maybe just go right away to Melbourne," Muguruza said. "I didn't expect to, you know, to not continue here."
Muguruza never got a chance to relax against Krunic's counter-punching, needing a 16-minute game that went to deuce 10 times to clinch the first set. She had the break in the second and was potentially two points from victory before Krunic went on a roll, leveling the match after Muguruza gave her three set points with a double-fault.
"It's definitely not a nice feeling, first of all for me as an athlete, to see my colleague walking out of the court not being able to finish the match," Krunic said in an on-court TV interview. "I want to wish Garbine a fast recovery and I hope she gets better for the Slam."
The result leaves defending champion Karolina Pliskova, who reached the No. 1 ranking last July before finishing 2017 at No. 4, as the highest-ranked woman in the tournament.
In men's first-round matches, U.S. qualifier Michael Mmoh beat Federico Delbonis 6-3, 6-4, Chung Hyeon upset fifth-seeded Gilles Muller 6-3, 7-6 (1) and Kyle Edmund rallied for a 6-7 (5), 7-6 (4), 6-4 win over Denis Shapovalov.