BRISBANE, Australia - Aleksandra Krunic continued her late-season momentum into the start of 2018, surviving top seed Garbiñe Muguruza, 5-7, 7-6(3), 1-2, advancing when the reigning Wimbledon champion was forced to retire at the Brisbane International due to muscle cramps.

"I felt in trouble in the second set when I was 2-0 up," Muguruza said of her fourth straight injury-induced finish in Brisbane. "I start to feel my calves were cramping. I continued to think that with the match they might go away, but then they were increasing, increasing. And then I had a lot of part of my body cramping.

"I cannot believe it. I don't know. It's a shame because I always come here excited the first tournament, and this one was bad luck, I guess."

Krunic kept things close throughout the two hour, 26 minute epic, leading by a break in the opening set only to roar back from a 2-5 deficit in the second to level the match as Muguruza struggled to remain optimal physicality on Pat Rafter Arena.

Following a stumble to start the third game of the decider, the former WTA World No.1 opted to end the match.

"It's not a nice feeling, first of all for me as an athlete, to see my colleague having to walk off court unable to finish a match," Krunic said during her on-court interview. "It's a disappointment. Even if I'm going to the next round, blah blah blah, the match wasn't over and there was still a lot of play to come from both of us and for the crowd.

"I'm not happy with what happened today and I want to wish Garbiñe a fast recovery. I hope she gets better for the Slam."

The Serb is in the midst of a career renaissance since last year's US Open, where she stunned Johanna Konta in the first round, and has now reached the quarterfinals or better of her last four tournaments dating back to the Guangzhou International Women's Open, where she finished runner-up.

"I didn't feel the heat that much today; that was probably because of the stadium. I'm sweating, but maybe I was too focused, thinking, 'I'm playing Muguruza, so I can't feel anything. I have to play with no feelings involved!'"

In all, Krunic struck 26 winners to 30 unforced errors, an impressively aggressive compilation for one who doesn't consider herself a big hitter.

"I tried to use all my weapons. I tried to make her play long points because I obviously couldn't hit with her. All the girls are double the size of me, so I can't outhit most of them. I tried to run, and to use my hands and my legs," she said.

"I'm happy I could put up a great fight because you never know what's going to happen in a third set. I'm happy I fought. I'm happy I really gave everything I had, and I'm happy that I was thinking right way and that I just stay in the match. I literally tried to do everything I could and tried to outrun her and make her play longer points, because I couldn't outhit her definitely."

Standing between the 24-year-old and a spot in the semifinals is the winner of the second round between No.7 seed Anastasija Sevastova and Sorana Cirstea.

Meanwhile, Muguruza's loss means she is no longer in the running towards becoming the WTA's next No.1 ranked player. Simona Halep can gaurantee she holds onto the top spot with a win over Duan Ying-Ying tomorrow at the Shenzhen Open; should the Romanian lose, Caroline Wozniacki can leapfrog both Halep and Muguruza by winning the ASB Classic in Auckland.

"I had some opportunities that I didn't turn, and it was just a tough match," the Spaniard concluded. "I felt a good level, good match, but I wanted to finish to see -- to evaluate how the match was, but I'm happy with the way I was playing."