LONDON, England - Kim Clijsters made the second week of Wimbledon on Friday afternoon, though it may not have been the way she wanted, when Vera Zvonareva retired in their third round match with a respiratory illness.

Zvonareva had been one of Clijsters' toughest opponents since the Belgian returned to the WTA in 2009 - one of only two players to beat her more than once, along with Victoria Azarenka. And the No.12-seeded Russian kept it close for 83 minutes before having to stop with Clijsters ahead, 63 43.

"It was very difficult for me to breathe and continue the match," Zvonareva said. "I've had it for four days, but it was getting worse and worse every day, and today it was pretty bad. I'm disappointed I couldn't continue the match.

"You never want to retire during a Grand Slam, and especially in a great match when you're playing against a good player. It never feels good to do that."

Zvonareva has had a difficult first half of 2012, retiring and withdrawing from several events with hip and shoulder injuries - she missed the French Open with the shoulder. She started the year No.7 and is currently down at No.12.

"When I'm not moving and not active physically, it's better," Zvonareva added about the illness. "Moving and running is just impossible right now - it's impossible to play at this level. But I hope it will get better soon."

Clijsters added the conditions made it difficult to play regardless. "It was tough to have a good feeling out there today," she said. "The wind made it really tough for both of us, and Vera hits a flat, low ball over the net, which is hard.

"But I did what I had to do out there. To say I'm coming off the court with a good feeling, I don't think so - but I have two days to practice and get that feeling."

Clijsters now has the weekend off before her fourth round match on Monday.

"I'm happy with the two day break," said the Belgian, who came into the tournament after an abdominal injury scare last week at the lead-up event in 's-Hertogenbosch. "With my stomach, every day of rest I have is a bonus."

Next up for Clijsters will be No.8 seed Angelique Kerber, who cruised past No.28 seed Christina McHale, 62 63. It was Kerber's WTA-leading 43rd match win of the year - Agnieszka Radwanska is next on the list with 41.

Kerber and Clijsters will be playing each other for the first time.

"I've never played against her," Clijsters said. "Obviously a left-hander is always tough. She's been playing some really good tennis. She's a fighter and physically strong girl, so it will definitely be a tough match for me.

"But I look forward to playing against somebody like that who I never played against before, where you just have to really be on top of your game, try to play aggressive tennis, but at the same time kind of get to know how your opponent plays and get a feel for it, the type of shots she makes, the types of decisions she makes out there. We'll see how it goes when we play on Monday.

"I'm excited I made it into the second week. A week ago I didn't expect I would do this, so I'm very happy. To still be in the tournament, to be a part of that second week at Wimbledon, it's always very special. It feels unique."