Defending champion Mikhail Youzhny made a first-round exit at the World Indoor Tournament in Rotterdam, losing 7-6 (7-3) 6-4 to Janko Tipsarevic.
Tipsarevic missed the recent Davis Cup tie through illness
The Russian fourth seed broke serve in the fifth game but unseeded Serbian Tipsarevic hit back to level before taking the tie-break.
Youzhny also dropped his serve in the first game of the second set as his opponent took charge at the baseline.
"Physically it was tough but mentally I stayed positive," said Tipsarevic.
The Serb reached the third round of the Australian Open last month but had to pull out of the Davis Cup tie with Russia in Moscow earlier this month due to a stomach bug.
"Mikhail is an aggressive player and I had to prevent him taking control," added Tipsarevic. "I just had to keep him moving."
Youzhny's compatriot Teimuraz Gabashvili had better fortune against Juan Carlos Ferrero, beating the former world number one eaten 7-5 6-1.
The seventh seed from Spain, now ranked 20th in the world, had little answer to the Russian qualifier.
"I didn't play my best tennis from the beginning and felt slow after I had stomach problems on Sunday," said Ferrero.
Unseeded Croatian Ivo Karlovic thumped 12 aces as he went through with a 6-1 6-2 victory over Olivier Rochus of Belgium.