PARIS -- Pete Sampras injured his back and withdrew from the Paris Open on Wednesday as Andre Agassi clinched the year-end No. 1 ranking for the first time.

Agassi, who was ranked 141st in the world nearly two years ago, becomes the 10th player to finish the year No. 1 since the rankings began in 1973. He defeated Younes El Aynaoui 6-4, 6-3 on Wednesday.

Winner of the French and U.S. Opens and a finalist at Wimbledon, Agassi clinched the ranking when Sampras pulled out of the Paris Open and Yevgeny Kafelnikov lost his second-round match at the indoor tournament.

Playing his first match since Aug. 20, when he hurt his right hip during a match at Indianapolis, Sampras beat Francisco Clavet 4-6, 6-3, 7-6 (7-4) Tuesday night.

Sampras wouldn't speculate on how long the back spasms will sideline him, although tournament physician Per Bastholt said he would be out 10-15 days.

Defending champion Greg Rusedski also pulled out of the tournament Wednesday after sustaining a leg injury while playing Albert Costa.

The Briton first hurt his leg at last week's Stuttgart Open, where he was knocked out in the semifinals by Richard Krajicek.

Sampras said the injury isn't as serious as the one just before the U.S. Open.

"The spasm is bad enough to stop me playing, but not so serious as the disc injury," he said.

Sampras has finished No. 1 for six straight seasons. He is ranked No. 3, trailing No. 1 Agassi and No. 2 Yevgeny Kafelnikov.

Agassi has a 1,115-point lead over Kafelnikov in the world rankings with just one tournament to be played before the year-ending ATP Tour World Championship in Hannover, Germany. He cannot be overtaken.

Sampras wants to play in Hannover, which begins Nov. 22.

"I haven't made any decisions," he said. "I will go home and play it by ear, but the World Championship is a huge event and I will do whatever I can to play. But my health is my job."

Sampras, who isn't assured a place in the World Championship even if fit, began to worry about his back during the middle of the second set.

"My left side got tighter and tighter. I felt it get worse and there was a complete spasm," he said.

Sampras, who herniated a disc Aug. 29 while practicing for the U.S. Open, clutched his back after a smash during the third set.

"I hit an overhead and felt it stabbing me. It was quite a scary feeling but I knew it was muscular and not the disc," he said. "When I got back to my hotel I had a hard time even getting around my room. Last night and this morning I reached the point of no return."

Sampras, a two-time champion of this tournament, said he considered defaulting against Clavet.

"But in a competition I have a lot of pride," he said.