Fromer Chmapions, Kim and Victoria forced to work for victory.

Defending champion Kim Clijsters triumphed on stadium court today over Maria Jose Martinez Sanchez in the third round of the Sony Ericsson Open, ringing up her 500th tour-level match win in the process. The World No. 2 fought for two hours to defeat the Spanish player 6-4 4-6 6-3.

Martinez Sanchez, a left-handed player and the No. 27 seed, challenged Clijsters with a powerful return game, and Clijsters had more unforced errors than usual. “I think a couple of times my footwork wasn't right there and I was not playing the game that I should be playing,” said Clijsters.

“It’s a little hard when you have a player who plays two totally different shots on each side,” she added, acknowledging the difficulties Martinez Sanchez’s left-handed game presented. Nevertheless, the Belgian player saved five of nine break points to close out the match with a total 33 winners.

Clijsters defeated Venus Williams to secure the title here last year, and also won in 2005 against Maria Sharapova, who advanced to the fourth round with her win yesterday.

Although Clijsters retired in the fourth round of Indian Wells last week with a shoulder injury, she seemed confident in her strength for Miami. “I have to focus a lot of my attention and my time on my shoulder, but I know if I can do that that it will stay stable,” she said.

The other former champion of the Miami tournament, Victoria Azarenka, defeated No. 25 seed Dominika Cibulkova 3-6 6-3 6-4. The tiny but powerful Slovak put up a fight against the No. 8 seed, who advances to play a fourth-round match with Pavlyuchenkova.

Cibulkova looked as if she would take the match in the first set, converting three break points and outplaying Azarenka, who had four double faults. The native of Belarus improved her game in the second set, where she broke at love and held at love to power into the third.

In the last hour of play, Azarenka had a read on Cibulkova’s service game to break and hold for a 3-1 advantage. Cibulkova would not be deterred, however, and evened the set at three all. She hit a backhand down the line to bring the final game to 30-all, but Azarenka pushed through the last two points to claim the match off a backhand winner. Cibulkova has not advanced past the third round in Miami.