David Ferrer survived a scare at the Geneva Open as Denis Istomin was forced to retire through injury in their second-round match.

Istomin was already a set and a break to the good on Wednesday when injury ended his match, handing Ferrer a reprieve and a place in the last eight.

However, having just broken Ferrer for a 3-0 lead in the second set, Istomin was unable to continue due to a back issue.

Thomaz Bellucci's hopes of regaining his 2014 title are over as he was eliminated in the second round, the Brazilian comfortably beaten 6-3 6-0 by sixth seed Federico Delbonis.

After a month out with a knee injury, John Isner made his return to the ATP Tour against Lukas Rosol. Although his serve worked well – he fired 29 aces – the American was unable to progress.

Isner started well as the only break of the match gave him the opening set, but Rosol prevailed in two tie-breaks to record a 5-7 7-6 (7-5) 7-6 (7-5) victory.

Rosol will meet Andrey Kuznetsov in the quarter-finals after he also beat an American, Steve Johnson joining Isner in dropping out in round two.

Third seed Marin Cilic is also in the last eight – where he will face Delbonis – while Guillermo Garcia-Lopez is Ferrer's next opponent.