The German retired on Wednesday from the WTA tournament in Adelaide when trailing Ukrainian Dayana Yastremska 3-6 0-2 in a second-round fixture on Wednesday.

Kerber's injury came as Australian qualifier Arina Rodionova's tournament ended in a second-round trouncing from Czech eighth-seed Marketa Vondrousova, who didn't drop a game in a 6-0 6-0 romp.

Vondrousova will now meet world No.1 Ashleigh Barty in a Thursday night quarter-final - a rematch of their French Open final last year when the Australian collected her maiden grand slam title with a 6-1 6-3 thumping.

"It is going to be tough," Vondrousova said.

"She is at home, world No.1 (but) I am really looking forward to it."

Vondrousova's victory came after Kerber - who triumphed at the Open in Melbourne in 2016, the same year she finished as world No.1 - sought on-court medical treatment for a lower-back injury before retiring.

The 31-year-old made the fourth round last year at the Australian Open but then struggled, losing first-round matches at the French and US Opens.

Kerber also lost in the first round against Australian Sam Stosur at last week's Brisbane International.

The diagnosis on Kerber is not immediately known but the world No.18 faces a race to be fit for the Australian Open which starts on Monday.

Ukraine's Yastremska advances to a quarter-final against Croatian Donna Vekic who downed Maria Sakkari of Greece, 2-6 7-5 6-1.

Second-seed Simona Halep will play Belarusian wildcard Aryna Sabalenka, who defeated American Bernarda Pera 7-6 (7-4) 6-2.

Amercian Danielle Collins, who upset her seventh-seeded compatriot Sofia Kenin, 6-3 6-1, will meet fourth seed Belinda Bencic, who defeated German Julia Goerges 7-6 (8-6) 7-6 (7-4).

Australian Associated Press