Teen Linda Noskova, Ukrainian qualifier Dayana Yastremska into AO quarters

Ukrainian No. 19 seed Elina Svitolina trailed 3-0 in the opening set and bowed out of the Australian Open with a back injury, pushing Czech teen Linda Noskova into the quarterfinals in upset-mad Melbourne.

"Today was not the way I've planned to win. I feel sorry for Elina and I hope she feels well very soon," said the 19-year-old Noskova, who embraced Svitolina at the net after a lengthy medical timeout signaled the end of their fourth-round meeting.

Noskova reached the quarterfinals of a Grand Slam for the first time in six tries.

She'll oppose Urkainian 23-year-old qualifier Dayana Yastremska in an all-underdog quarterfinal. Yastremska beat two-time champion Victoria Azarenka 7-6(6), 6-4 only 30 minutes after Noskova's match went final.

"My heart is going to jump out of my body," Yastremska said post-match, her seventh consecutive victory since qualifying began.

Azarenka went ahead 3-0 in the second set. Yastremska admitted she was feeling fatigued, but found another gear.

"My coaches were telling me 'play in 2 shots,' " Yastremska said.

"I'm like ‘how can I play in two shots?' Vika's running good. Then I just started: boom, boom, boom. I don't know how to explain it."

Noskova entered the field ranked 50th and already eliminated No. 1 Iga Swiatek, who called the big-serving Noskova "fearless."

She's the first teenager to beat a world No. 1 at the Australian Open since Amelie Mauresmo in 1999.

Svitolina cruised into the matchup by blowing past Viktorija Golubic, 6-2, 6-3, in the round of 16.

Trailing 2-0 in the opening set, Svitolina unfurled a towel and laid face-down hoping her back would respond during a medical timeout. Her injury was evident during attempts to serve, writhing and grimacing without the usual pop from the racket.