SYDNEY—Ons Jabeur retired with a lower back injury in the Sydney Classic quarterfinals on Thursday, allowing Anett Kontaveit to advance. The fourth-seeded Kontaveit advanced after the seventh-seeded Tunisian lost the opening set 6–4 and decided to stop soon after. Jabeur received a medical timeout after the set and then attempted to start the second set. But after serving a let and a first-serve fault, she walked to the net for a match-ending hug. She is seeded ninth for the Australian Open, which starts next week. Jabeur reached the quarterfinals at Melbourne Park in 2020. “I really hope that Ons is going to be fine for next week and that it’s nothing too serious,” Kontaveit said. “The first set was really competitive and we’ve had some tough matches before, but she’s always come on top. I was really happy to get the set and that I was playing the way I …