Top seeds Roberto Bautista Agut and Jo-Wilfried Tsonga battled into the St Petersburg Open quarterfinals but Philipp Kohlschreiber was forced to withdraw.
The German fifth seed did not play a point of his round-of-16 encounter with Ricardas Berankis as illness prevented him from taking to the court - but other results went in favour of the more fancied players.
Bautista Agut overcame a second-set wobble against Rogerio Dutra da Silva to demolish the final set with a 'bagel' and win 6-4, 4-6, 6-0, while Tsonga also needed three sets to put away Portuguese Joao Sousa 7-6 (7-1), 3-6, 6-2.
Fabio Fognini of Italy got the better of Mikhail Youzhny in a finely balanced match that ended 6-4, 4-6, 6-4, while Viktor Troicki beat Guido Pella 7-6 (7-3), 4-6, 6-4 to set up a meeting with Bautista Agut.
Tsonga will take on German Jan-Lennard Struff -- who beat Thomas Fabbiano 6-1, 6-4 in his last-16 match - in Friday's quarterfinal action, which will be opened by Briton Liam Broady's match against Damir Dzumhur.