They Are Down to the Class on Grass : Wimbledon: It is catch-up day today with men’s quarterfinals and women’s semifinals.

The rain-plagued Wimbledon tennis tournament that some thought might never end goes into its busiest--and probably most exciting--day today.

The women will play the semifinals, with Steffi Graf meeting Mary Joe Fernandez and Jennifer Capriati facing Gabriela Sabatini. The women’s final will be Saturday.

The men will play the quarterfinals, with Stefan Edberg going against Thierry Champion, Jim Courier against Michael Stich, Andre Agassi against David Wheaton and Boris Becker against Guy Forget.

Following is a capsulized look at this select group of 12:

WOMEN

No. 1 Graf vs. No. 5 Fernandez--Graf looks like the Graf of old, the two-time champion here, who seemed to lose her concentration last year when her father was accused of--and eventually admitted to--cheating on her mother. This year, with family problems and missed forehands behind her, she looks capable of winning again. She has lost a total of 17 games in five matches and is, once again, hurrying through matches like somebody who is afraid to miss the last bus. Fernandez is in the Wimbledon semifinals for the first time, and while she is playing very well and took out favored Aranxta Sanchez Vicario in the quarterfinals, she has faced Graf five times and has yet to win a set.

No. 2 Sabatini vs. No. 9 Capriati--This one could be a dandy, because Sabatini is the hottest player on the tour this year, with a 45-4 record; and Capriati, with her stunner over Martina Navratilova Wednesday, is the hottest player in the women’s draw. Sabatini has won one Grand Slam title, last year’s U.S. Open, and has been in the Wimbledon semifinals twice. She also has played Capriati five times and never lost.

MEN

No. 1 Edberg vs. unseeded Champion--Edberg has won the title two of the past three years here, and he appears to be playing even better on grass than in the past. Like top-seeded Graf on the women’s side, Edberg, who took out John McEnroe in the fourth round, has yet to lose a set. Frenchman Champion, whose idol is Bjorn Borg, may never again get this close to emulating what his idol did on these grass courts. Champion’s favorite surface really is clay, and he never has gotten past the first round here. Just about everybody is surprised that he got this far. He did so by beating ace grass player Derrick Rostagno Wednesday, in a match held over from Tuesday.

No. 4 Courier vs. No. 6 Stich--They have played three times this year, including a semifinal match in the recent French Open, when Courier was en route to his first Grand Slam title, and Courier has a 3-1 record in those matches. But Stich is a big server; Courier has said that he has surprised himself by getting this far, and today’s opening match on Centre Court will be Courier’s first match there. Courier said: “A couple of years back, Pete Sampras and I were over here for the Masters Doubles, and we were practicing indoors. We just came in to Centre Court and walked around, sat in there and kind of daydreamed a little.” Only three times has the French Open champion gone on to win at Wimbledon in the same year. Borg did it twice and Rod Laver once. Stich won a five-setter Tuesday over Alexander Volkov to reach the quarterfinals.

No. 5 Agassi vs. unseeded Wheaton--The buzz over Agassi’s appearance here, after he stayed away since a first-round loss to Henri Leconte in 1987, has not gone away. He has gathered up both new fans and new groupies, and those who said he would not be able to play well on grass are being proved wrong. Wheaton, a tall, big server, took out Ivan Lendl in the third round and has made his way into the quarterfinals. He has played 15 sets so far and eliminated Jan Gunnarsson in his only easy match Tuesday, in straight sets. Wheaton and Agassi have played three times, Wheaton winning twice. He has the kind of game to give Agassi trouble on grass. Agassi, in yet another appearance on Centre Court Wednesday, beat Jacco Eltingh, an unheralded player from the Netherlands, 6-3, 3-6, 6-3, 6-4.

No. 2 Becker vs. No. 7 Forget--Forget, a fast-rising Frenchman who, unlike most of his countrymen, has a serve-and-volley game better suited for grass than for the red clay of his beloved Roland Garros Stadium, has played Becker seven times and won only once, in 1984, when Becker was 17. Forget barely survived a five-setter on a back court against Gilad Bloom in the first round, then got a break in the third round when fellow Frenchman Henri Leconte, still in the match, had to default because of a sore back. Becker is Becker. He won Wimbledon titles at 18, 19 and 22--the last two years ago--and has been in the final the past three years with Edberg. Forget got to the quarterfinals by beating veteran Tim Mayotte, 6-7, 7-5, 6-2, 6-4, Wednesday. Becker defeated Christian Bergstrom, 6-4, 6-7, 6-1, 7-6.

The winner of the Edberg-Champion match will play the winner of the Courier-Stich match, and the Agassi-Wheaton winner will face the Becker-Forget winner.