Former Australian Open champion Angelique Kerber sustained a lower back injury and retired from her Adelaide International match 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. 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.