Former Australian Open champion Angelique Kerber suffered a back injury days before the year's first Grand Slam, with the German forced to retire at the Adelaide International warmup on Wednesday.
The former world number one had a medical time-out for a trainer to work on her lower back before she conceded her second round match trailing 6-3 2-0 against Dayana Yastremska at Memorial Drive Tennis Centre.
Kerber, who upset Serena Williams to win the 2016 title at Melbourne Park, lost in the first round of the Brisbane International earlier this month.
The 19-year-old Yastremska will meet Donna Vekic in the quarter-finals after the Croatian beat Greek Maria Sakkari 2-6 7-5 6-1 in their earlier match.
(Reporting by Ian Ransom; Editing by Kim Coghill)
