Marcelo Rios and Michael Chang both advanced at the French Open today when their opponents retired in the middle of matches because of injury.

The third-seeded Rios, who could overtake Pete Sampras as the No. 1 player in the world if he makes the semifinals, became the first man to reach the fourth round. He was leading 6-1, 3-3 when Wayne Ferreira twisted his right ankle and was unable to continue.Ferreira was running for a shot along the baseline when his right foot appeared to get stuck on the clay. He twisted his ankle, grimacing as he fell on his back.

The South African originally hurt the ankle while practicing between rounds at last year's French Open and had surgery on it to remove fluid in October. He was taken to a hospital after the match for X-rays, and the extent of today's injury was not immediately clear.

Chang, the only American man left from the dozen that began the French Open, reached the third round when John Van Lottum of the Netherlands retired in the third set.

Chang, winner of this tournament in 1989 and seeded 11th, was leading 7-5, 6-2, 3-0 when Van Lottum was unable to continue because of a strained buttock muscle that he originally hurt in the first round.

Jan-Michael Gambill, the only other American to make it to the fifth day of the tournament, was eliminated in a second-round match that began Thursday night but was halted by darkness.

Among the other U.S. losers have been top-seeded Pete Sam-pras, two-time champion Jim Courier and two-time finalist Andre Agassi.

"I think the French Open has been a tournament that's been a little bit frustrating for American players, particularly on the men's side, because the depth is so great here and so many players are in such great shape and they know the surface so well," Chang said.

"Generally speaking, the U.S. players grow up on hard courts. I would suggest that for most Europeans, most South Americans, they really grow up on clay. Naturally, they're a little bit more comfortable on this surface. It takes them a lot less time to adjust to it."

Also advancing to the third round today were two Spaniards, No. 14 Alex Corretja and No. 16 Alberto Berasategui.

But No. 4 Patrick Rafter was ousted 6-4, 2-6, 6-3, 6-2 by fellow Australian and friend Jason Stol-ten-berg, leaving Rios as the only survivor among the top seven men's seeds.

Rafter, the defending U.S. Open champion, yelled "No, no, no!" at himself and stormed around the court as he blew opportunities in the third set. He slammed his racket to the clay several times in disgust.

"I was very frustrated. I wasn't winning games easily on my serve," Rafter said. "But I couldn't have lost to a better bloke, I'm very happy for Jason."