It was not immediately clear why Wawrinka abandoned against the 193rd-ranked Belarusian, but the three-time Grand Slam champion still appears to be struggling with his troublesome left knee and looked upset as he left the court. 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.

Ivashka advances 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.