Associated Press

MELBOURNE, Australia — Top-ranked Ash Barty built a big lead in her quarterfinal match at the Australian Open before her opponent took a medical timeout and left the court.

More than an hour later, it was Barty heading abruptly through the exit. She was upset Wednesday by Karolina Muchova, 1-6, 6-3, 6-2.

Seeded No. 25, Muchova earned her first semifinal berth in a Grand Slam. Her comeback win ended Barty’s bid to become the first Australian woman to win the title in Melbourne since Chris O’Neil in 1978.

“It’s heartbreaking, of course,” Barty said. “But the sun will come up tomorrow. You’re either winning or you’re learning, and today is a massive learning curve for me.”

Muchova’s opponent Thursday will be Jennifer Brady, who has reached her second Grand Slam semifinal in a row by beating fellow American and good friend Jessica Pegula 4-6, 6-2, 6-1.

The United States will have a semifinalist in the women’s draw at Melbourne for the 11th year in a row.

The No. 22-seeded Brady struggled at times and gave her racket an angry toss midway through the second set. The unseeded Pegula, who advanced beyond the third round at a major for the first time, appeared to tire down the stretch.

Brady is a former UCLA star. She reached her first major semifinal at the U.S. Open in September..

Muchova played poorly at the start of her second major quarterfinal, and Barty raced to a 5-0 lead while losing only six points. After nine games, Muchova had one winner and 18 unforced errors, and early in the second set, she took a medical timeout that lasted nearly 10 minutes.

“I started feeling a bit lost,” Muchova said. “Ash started very good. She played almost like no mistakes; it was very tough. I was a bit lost on the court and my head was spinning, so I took a break. And it helped me.”

Medical personnel took her temperature, checked her blood pressure and gave her ice before she left the court. When the Czech returned, she played much better. “I tried to play a bit faster rallies so we don’t play long ones as in the first set,” Muchova said, “and it worked well.”

Barty had no complaints. “It’s within the rules,” Barty said. “She’s within her rights.”