Nikolay Davydenko and David Ferrer, seeded two and three respectively, were knocked out of the World Indoor Tournament on Thursday.
Unseeded Frenchman Michael Llodra reached the quarter-finals by defeating Russian Davydenko 6-3, 7-5 while German qualifier Mischa Zverev ousted Spaniard Ferrer 6-2, 7-5.
Llodra, who won the Adelaide International title in January, avenged his first-round defeat by the world number four at last month's Australian Open.
Davydenko was hampered in the second set when he needed a timeout for treatment on his right shoulder and elbow.
"That has all to do with the heavy balls they use over here which are the same used at the US Open," the Russian told Reuters. "I felt the problem in training sessions and it has to do with the vibration the balls cause.
"There are other players complaining about it. It is sad I lost today but it will give me extra days to recover before the Dubai Tennis Championships (starting on March 3)."
Unseeded Croatian Ivo Karlovic hit 17 aces as he beat fifth-seeded Czech Tomas Berdych 6-4, 6-4. He next meets Zverev.
