MARSEILLE, France (AP) Second-seeded Stan Wawrinka hobbled out of the Open 13 second round with a recurrence of his left knee problem, retiring when trailing 6-4, 1-1 against Ilya Ivashka on Thursday.
After abandoning against the 193rd-ranked Belarusian, the three-time Grand Slam champion - who looked upset when leaving the court - said he struggled with his troublesome left knee from 3-3 in the first set onward.
''At 3-3 I hit a forehand volley and it hurt the knee a bit. I felt that it was very tense,'' he told the tournament organizer's website. ''After that I simply couldn't flex it.''
This loss follows the 32-year-old Swiss player's unexpected first-round defeat to 259th-ranked Dutchman Tallon Griekspoor at the ABN AMRO World Tournament in Rotterdam last week.
Wawrinka ended his 2017 season in August because of an injured left knee that needed surgery. Making his return, he lost in the second round to Tennys Sandgren at the Australian Open in January.
Wawrinka accepts he is still struggling and that he needs more time before regaining full fitness.
''When the knee becomes tenser, when it puts pressure on the knee cap, you shouldn't force it,'' Wawrinka said. ''That's when you make things worse. These are difficult times. It's a long process but you need to take a step back from everything that's happening.''
Ivashka advanced to the quarterfinals against Frenchman Nicolas Mahut, who upset sixth-seeded Gilles Muller of Luxembourg 3-6, 6-2, 6-4.
Meanwhile, Tomas Berdych rallied to reach the last eight. The fourth-seeded Czech labored to a 5-7, 7-6 (4), 6-3 win against unheralded Italian Stefano Travaglia, who had 18 aces. Berdych next plays seventh-seeded Damir Dzumhur of Bosnia-Herzegovina
Eighth-seeded Filip Krajinovic of Serbia and ninth-seeded Russian Karen Khachanov also advanced, but far more easily in straight sets.
Krajinovic won 6-3, 6-3 against French veteran Gilles Simon. Krajinovic was a surprise runner-up at the Paris Masters in November after coming through the qualifiers, and he next faces third-seeded Frenchman Lucas Pouille.
The 21-year-old Khachanov was in clinical form in a 6-2, 6-1 win against Mischa Zverev, taking all four of his break-point chances on the German player's serve.
He next plays 36-year-old Frenchman Julien Benneteau.
