Victoria Azarenka, the women's world number two, is one of five players who have pulled out of Wimbledon today, after failing to overcome a knee injury. 

The two-time semi-finalist fell heavily during her first-round match with Maria Joao Koehler of Portugal on Monday, and although she recovered to comfortably win the match 6-1 6-2, she has struggled with her recovery. 

Steve Darcis, the conqueror of Rafael Nadal, has also pulled out with a shoulder problem.

Number 10 seed and Queen's finalist Marin Cilic has quit with a knee injury, while John Isner and Radek Stepanek were forced to retire shortly after the start of their respective second round matches. 

Azarenka received lengthy on court treatment and heavy strapping after her fall, but the prospect of facing Italian Flavia Pennetta on centre court was not something she could face. 

"As soon as I went out I felt it," Azarenka told reporters. 

"This kind of injury takes about 48 hours to show if it's going to get better or worse. In my case it has gone worse there is nothing really positive to say and I have to deal with it.

"I have to have a further examination - my scan shows bruising but my whole body has had to compensate from the fall."

Darcis, who was scheduled to play Lukasz Kubot of Poland, announced his withdrawal on Twitter: "Having to throw in the towel after beating Rafa?! THE hardest decision of my career!!! #triedmybestanyway!!!"

Neither Darcis or Azarenka took to the court for their second round matches.

"It happened against Rafa in the middle of the first set when I fell down," added Darcis.

"After the (Nadal) match, a few hours after, I start to feel so much pain, I couldn't sleep that night. I saw the physio and the doctor yesterday. They did a good job. It's a little bit better today. But no chance I can play. I mean, I cannot serve. Even on the forehand side, I cannot hit a ball. It makes no sense to go on the court to withdraw after two games."

Both Isner and Stepanek required treatment during their second round matches but failed to recover enough to continue.

American Isner had played just two games of his match against Adrian Mannarino of France when he was forced out with an injury to his left knee.

"Always serve and land on my left leg, like I have done 20 million times playing this game, and this is the first time I just felt this, like, sharp pain," he said to reporters.

"It wasn't like a pop... it just grabbed really badly, and I knew I was in serious trouble then. I knew at that point it was not likely I was going to be able to play."

Stepanek soon followed, trailing one set to love and 5-3 to 24th seed Jerzy Janowicz, when he retired with a hamstring problem.

Cilic made it five withdrawals when he pulled out ahead of his second-round match with Frenchman Kenny De Schepper.

The world number 12 could have met Andy Murray at the quarter-final stage.

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