Angelique Kerber suffered an injury setback ahead of the Australian Open after pulling out of the Adelaide International warm-up tournament on Wednesday.
The former world No. 1 trailed 6-2, 2-0 in her match against Dayana Yastremska when she stopped playing.
Kerber, who won the Australian Open in 2016 and also has Wimbledon and U.S. Open titles, had on-court medical help for the injury and then retired.
Kerber lost in the first round to Sam Stosur last week at the Brisbane International.
In other second-round women's matches in Adelaide, Donna Vekic beat Maria Sakkari 2-6, 7-5, 6-1 and American Danielle Collins upset her seventh-seeded compatriot Sofia Kenin 6-3 6-1, to secure a quarterfinal spot.
In the men's ATP tournament at the same venue, third-seeded Andrey Rublev beat Sam Querrey 6-3, 6-3.
Additional reporting by PA.
