Top seed Mikhail Youzhny retired with a back injury midway through his second-round match against Denis Istomin at the St Petersburg Open on Friday.
Youzhny, fresh from winning his first Kremlin Cup title on Sunday, took a medical timeout after losing the first set but was forced to quit three games later, trailing 6-2 2-1.
"I just felt a sharp pain in my back midway through the first set," the Russian world number 25 told a news conference.
"I had to battle through the pain for the rest of the first set. The medical break didn't help either and soon I knew that I couldn't go any longer and had to stop.
"I just hope it's not too serious," added Youzhny, who has been suffering from back pains for much of the season.
Fifth-seeded compatriot Igor Andreev followed Youzhny out of the indoor tournament when he retired with a knee injury in the second set of his second-round encounter against Marat Safin, trailing 6-3 0-1.
"It's an old injury, I've had it on and off for the last few months," 38th-ranked Andreev told a news conference.
"I started feeling some pain in my left knee even before the match but I thought I would still be able to play. In the end, I just couldn't do it any more."
Safin said he was unaware his opponent was injured.
"I was just concentrating on my game and didn't know he was feeling pain in his knee," said the former world number one, playing in his penultimate tournament before retiring.
"In any case, I'm just happy I got through to the next round," added the Russian, who will now take on 101st-ranked Istomin of Uzbekistan in the last eight.
