Top seed Nikolay Davydenko retired from his second-round match against South Africa's Wesley Moodie with a foot injury while leading 6-2 3-3.
The Russian was joined on the sidelines by compatriot and eighth seed Mikhail Youzhny, who just 30 minutes earlier pulled out of his doubles match after spraining his right ankle. The injury handed Swede Thomas Johansson a walkover into the quarter-finals.
Earlier in the day, fifth-seeded Finn Jarkko Nieminen also retired from his second-round match against Potito Starace with a foot injury, with the Italian leading 7-5 3-2.
"I hurt my right foot in the seventh game of the first set when I ran for a short ball," Davydenko told a news conference.
"I just felt a huge pain in my foot. I don't think it was my ankle, it was more like my heel.
"I called a trainer after the next changeover to tape my foot.
"I won the first set but in the second I felt more pain and had trouble moving around the court.
"I probably could have continued but I wasn't sure how I would feel tomorrow and I just didn't want to take any chances with a Masters tournament in Paris coming up next week and the (season-ending) Masters Cup in Shanghai to follow."
Johansson, who won here last year, will now meet Moodie in Friday's quarter-finals.
YOUZHNY'S INJURY
Youzhny's injury appears to be more serious.
The world number 23 was taken to a local hospital for x-rays.
"Medical test will show if Youzhny would be able to participate in any future tournaments for the rest of the year," ATP officials said in a statement.
The 97th-ranked Starace advanced to the quarter-finals for the third time this year and will meet either fourth-seeded German Tommy Haas or Russian wildcard Igor Kunitsyn.
Sixth seed Dmitry Tursunov was shown the exit after losing to Belarus' Max Mirnyi 4-6 6-3 6-4 in the first round.
Mirnyi broke the Russian in the seventh game of the final set when the world number 22 double-faulted on break point.
The lanky Belarussian then closed out the match with his customary delivery in just under two hours.
Haas also booked his place in the second round with a 6-2 6-7 6-2 victory over Serbian qualifier Nenad Zimonjic.
The German allowed a 4-1 lead in the second set to slip through his grasp before making amends in the decider.
"Finally, I got to celebrate my first win here today," said Haas, who lost in the first round on his previous visit in 2001.
"You never know what to expect from a player like Zimonjic, who is mostly known as a doubles player, especially playing him indoors so I was quite happy with my game today.
"I've made a few easy unforced errors at the end of the second set but fortunately I was able to find my rhythm again in the third set to close out the match."
