BRISBANE: 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.
Krunic advanced to a quarterfinal match against either seventh-seeded Anastasija Sevastova or Sorana Cirstea.
The Brisbane International is one of the main tuneup events for the Australian Open in Melbourne, the first major of the season beginning Jan. 15.
"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."
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.
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.