NOTTINGHAM, England (AFP) - Radek Stepanek faces a race against time to be fit for Wimbledon after the Czech pulled out of the Nottingham Open with a rib injury.

Stepanek, 29, was due to play Vince Spadea on Wednesday but was forced to withdraw when his physio discovered a muscle tear suffered in the previous round.

He will now have scans to determine the extent of the problem and whether he will be fit for Wimbledon, which starts on Monday.

Stepanek, the world number 15, said: "Yesterday at the end of the match against (John) Isner I pulled a little bit the muscle under my rib but I didn't pay much attention.

"I finished the match and I wanted to go and practice but when I came on the court I felt a pain under my rib and I went to see the physio and he said I have a muscle tear there.

"This morning, after I hit five shots, I had a really bad pain there, like somebody sticking a knife in there, and that's why I had to pull out because I'm not able to compete."

Defending champion Ivo Karlovic won a mammoth second-set tie-break against American Sam Querrey to reach the quarter-finals with a 7-6 (7/3), 7-6 (16/14) victory.

The 6ft 10in (2.10m) Croatian looked to have the measure of his opponent in the first set but Querrey more than matched him in the second and had four set points in the breaker.

Karlovic missed four match points -- including two on his own serve, one of which was a double fault -- before eventually sealing victory on the fifth.

Karlovic, who hit 27 aces, felt he took his opportunities on the Querrey serve at the right time. He said: "It was difficult. We play a pretty similar game, he serves well so it's difficult to hit returns but I was able to put in a few good returns in the tie-breaks when it was important.

"It was windy so it was difficult to serve but I was focused throughout the whole match. "It's something players get used to and ignore."

French Open semi-finalist and eighth seed Gael Monfils beat French countryman Julien Benneteau 3-6, 6-4, 6-2 and now faces Spadea while seeds Andreas Seppi of Italy, France's Gilles Simon and Fernando Verdasco of Spain also reached the last eight.