When Dick Stockton arrived here two weeks ago, his right shoulder stiff from a lingering case of tendinitis, he had only modest expectations for himself in the French Open tennis championships.

"I really had planned on using this only as a tuneup for Wimbledon," the 6-foot-2 New Yorker-turned Texan said yesterday after upsetting one of the clay court masters, Spaniard Manuel Orantes, 5-7, 6-3, 6-4, to reach the semifinals of the world's premier clay court test.

"After the WCT finals (in Dallas last month), I went to the beach and didn't play any tennis until coming here four days before the tournament to practice on the clay . . . I hadn't played well for several months. I just eanted to get in good shape before Wimbledon."

Stockton served well and played thoughtful, resourceful tennis to take the initiative away from Orantes and record what he called "my best clay court win and effort."

He will have a two-day rest before meeting defending champion Guilermo Vilas, who yesterday found himself in the unfamiliar role of aggressor against the unorthodox Hans Gildemeister of Chile.

Vilas, though still not as sharp as last year when he was the world's dominant clay court player, ultimately was too strong for Gildemeister and won, 6-4, 2-6, 6-1, 3-6, 6-3, in a backcourt battle of attrition that delighted a crowd of 13,475 at Stade Roland Garros.

Bjorn Borg, the No. 1 seed and champion here in 1974-75, plays Raul Ramirez of Mexico, and Eddie Dibbs of Miami opposes Iralian Corrado Barazzutti, in the other quarterfinal matches today.

Stockton 27, is playing in Paris for only the second time.In 1973 he lost in the fourth round to Borg - "When he was 12," Stockton deadpanned yesterday.

Stockton nearly lost in the third round this year. He was down two sets to one and 1-4 in the fourth set, a point away from 1-5, against Hungarian Balazs Taroczy. He survived two match points before winning that match with some bold and gutsy shotmaking.

Yesterday's match against Orantes was inspiring only in the fourth set. And that is when Stockton played his most aggressive tennis by challenging Orantes' superior ground game by jumping on short balls and getting to the net behind deep approach sthots.

"I've been serving well since I got here. I just wanted to make sure I got my first serve in and moved the ball around in the rallies until I got a chance to go in," said the Trinity (Texas) University grad who now lives in Dallas.

Stockton started slowly. This was his first match in five years on the center court, and it took him some time to get adjusted.

"I was hitting everything long because the back ground is so much bigger than on the outside courts," he explained. "At the beginning, I didn't have any depth perception."

Stockton lost his serve to 2-3 in the first set, caught up at 4-4, then was broken again in the ninth game, making two unforced errors around two superb passing shots by Orantes.

But the graceful and even-tempered Spanish left hander, runner-up here in 1974 and U.S. Open Champion in 1975, was not hitting his ground strokes with his customary depth or accuracy. He couldn't keep the pressure on, and was never again up a bread in any set.

Orantes' forehand was erratic, and he could not fend off Stockton's deliberate, well-executed advances to the net. He hit a few lovely topspin lob winners, but in the crunch he was lobbing too shallow to scap Stockton's punishing overhead.

Both raised their games in the final set. Orantes lost his serve in the first game, but broke back to 2-2 with a winning lob in a game in which he took a nasty tumble and came up grimacing rubbing his back.

Orantes defaulted to Vilas in the semifinals of the German Open and withdrew from the Italian Open because of back spasms. Stockton, whose career has been plagued by chronic lower back problems, came flying over the net to see if Orantes was all right.

He was, holding his serve at love in the fifth game and at 30 in the seventh as both players dug in and played some fine points. But Orantes lost his serve in the ninth game, after two duces and two game points, netting a forehead following a beautiful backhand down-the-line pass by Stockton.

Stockton then held at 15 for the match, Orantes saving one match point. It was a satisfying victory for the American, who started off well this year by reaching the final of one tournament and winning another before being sidelined for five weeks by his bad back.

"I had to default from a tournament in St. Louis in February when my back went out, and after that every thing just fell apart," Stockton said. "I never found my timing and rhythm again until I got here."

Vilas came to the net much more than usual in the marathon against Gildemeister, a lanky former University of Southern California player who hits two-fisted off both sides and blasts outstanding passing shots, especially sharply angled cross-court shots.