To the French, the two weeks of the French Open are a celebration. Summer has virtually arrived, the sun stays out until 10 p.m. and the greatest names of a game they adore take up residence in downtown Paris.

Today, the French had ample reason to celebrate.

They cheered lustily when 19-year-old Pascal Paradis, playing her first match on center court at Roland Garros, opened the tournament on a cool, breezy day in Paris by upsetting 15th women's seed Andrea Temesvari, 7-6 (7-4), 3-6, 6-3. They were almost as delighted when Henri Leconte, a dashing 21-year-old, ousted Tim Wilkison, 7-5, 6-2, 6-1. And they were almost delirious when Tarik Benhabiles came from two sets down late in the day to beat another American, Todd Nelson.

Even Thierry Tulasne's straight-set loss to fourth-seeded Mats Wilander did not dampen the enthusiasm of the French, who burst into rhythmic applause the one time Tulasne broke Wilander's serve.

All this and national hero Yannick Noah, the 1983 champion here, doesn't begin play until Tuesday.

"Having the French people behind me made a big difference for me today," said Paradis, who survived two break points at 2-3 in the third set and never lost another game. "In the U.S. Open when I was leading (Carling) Bassett 5-1 (and a set) in the second, I was still nervous and I ended up losing. Today, I was never nervous because the crowd was with me."

Among the women, Paradis was the only major upset winner as top-seeded Martina Navratilova took 35 minutes to dispose of Pam Teeguarden, 6-1, 6-0, and lead a group of seeds that included No. 3 Hana Mandlikova, No. 6 Zina Garrison, No. 8 Bassett, No. 13 Kathy Rinaldi and No. 14 Gabriela Sabatini into the second round. All won in straight sets. Only Bassett, forced to two tie breakers by Andrea Betzner, had trouble.

The same could not be said for the U.S. men. No American has won the men's side of this tournament since 1955 and today, with top-seeded John McEnroe and No. 3 seed Jimmy Connors yet to play, the Americans suffered on the slow clay that makes Roland Garros almost unique.

Eighth-seeded Eliot Teltscher won, defeating Dacio Campos, 6-2, 6-3, 6-3, but eight other U.S. males lost, some of them shockingly. Most notable was 15th-seeded Brad Gilbert, the only male seed to lose today. Gilbert botched a set point in the first set, lost a tie breaker in the second and was swept away, 7-5, 7-6 (7-5), 6-4, by Chile's Hans Gildemeister. Now 30, Gildemeister has reached the quarterfinals here three times, but the last time was five years ago.

Today, though, his steadiness and familiarity with the clay were the difference as he won over a player ranked 74 spots ahead of him on the ATP computer.

Gilbert's defeat was not the most shocking of the day. Although Gildemeister's best tennis is behind him, he is a clay court player of proven quality. The same cannot be said of Florin Segarceanu, the Romanian (ranked 188) who beat Paul Annacone, 7-5, 6-7 (7-4), 7-6 (7-4), 6-2.

Annacone, a 1984 Wimbledon quarterfinalist, played here against the advice of his college coach, Mike DePalmer, who warned him about the difficulties of the clay. Annacone double-faulted to lose the third-set tie breaker and Segarceanu easily put him away in the last set.

"You could almost see him saying 'What am I doing out here?' ," said U.S. Davis Cup Captain Arthur Ashe, who watched Gilbert and Annacone lose. "You just can't play your whole life on (surfaces other than) clay and then come over here and do well on the stuff. The clay court players are willing to stay out there all day if they have to. Our guys aren't used to that. It's discouraging to watch."

Ashe would have been even more discouraged had he walked over to court No. 3 to watch yet another American, Mark Dickson, lose a remarkable five-set match to Australia's Darren Cahill, 3-6, 6-2, 6-1, 2-6, 14-12.

It took just under four hours for Cahill to pull his upset, perhaps the day's most stunning in light of the fact Cahill, 19, got into the draw as a qualifier; is ranked No. 266; had never played a five-set match and didn't even know that there is no tie breaker in the fifth set here.

After serving in that last set to even the match at 6-6, Cahill trudged to the base line, looked at the umpire and said, "Tie breaker?"

"Non, monsieur," he was informed. Cahill shrugged and dug in. "I wasn't sure," he said afterward. "I figured I better check."

With the crowd that overflowed the stands on the side court very much on his side, Cahill traded service games with Dickson through 12-12. Neither player had a break point.

Finally, with Dickson serving in the 25th game, Cahill got to 15-40 with a gorgeous cross-court forehand. Dickson, tugging on his ever-present cap, got a first serve in to save one break point. On the second, Cahill slammed a forehand pass down the line and then fell flat on his face as screams of "Bravo!" came from exhausted but delighted spectators.

Dickson wasn't quite finished. He hit two backhand winners and had a break point when Cahill netted a backhand volley. But Cahill hit three clean winners, the last a punched backhand volley, and threw his arms high in the air, overjoyed by his victory.

Virginia Wade, the former Wimbledon champion who is now 39, defeated Sarah Gomer, 7-5, 6-4. But life, even on a day of celebration in Paris, is never perfect. As a reward, Wade gets to play Navratilova.