Former Australian Open champion Angelique Kerber might not play next week in Melbourne after suffering a back injury at the Adelaide International.

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.

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 and 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.

Her opponent Yastremska advances to the quarter-finals of the inaugural Adelaide tournament, joined by Donna Vekic. The Croatian has downed Maria Sakkari of Greece, 2-6 7-5 6-1.

Also on Wednesday, Amercian Danielle Collins upset her seventh-seeded compatriot Sofia Kenin, 6-3 6-1, to secure a quarter-final slot.

Australian qualifier Arina Rodionova later plays Czech eighth seed Marketa Vondrousova to determine who meets world No.1 Ashleigh Barty on Thursday night in a quarter-final.