It began rather innocently shortly after 11 this morning when Carling Bassett broke Hana Mandlikova's serve in the first game of their quarterfinal match.

By the time it was over, the placid, true-to-form nature of the U.S. Open had been torn apart as bright sun and upsets returned to the National Tennis Center.

First, it was Bassett, the 16-year-old Canadian who missed most of the summer with mononucleosis, sweeping the third-seeded Mandlikova out of the tournament with a 6-4, 6-3 victory. The 58-minute match was over so quickly that many in the day crowd hadn't even arrived before Mandlikova was gone.

Within 30 minutes of Mandlikova's exit, No. 4 seed Pam Shriver also was gone, the victim of her own expectations and Wendy Turnbull. The 31-year-old Australian, a finalist here seven years ago but seemingly past her prime, stunned Shriver, 2-6, 6-3, 6-3, in a match that left Shriver screaming to the heavens in disgust.

Finally, four hours later, 19-year-old Pat Cash proved definitively that his trip to the semifinals at Wimbledon this year was no fluke, serving fourth-seeded Mats Wilander right out of town in a 7-6 (7-3), 6-4, 2-6, 6-3 victory. Cash is the first Australian in 10 years to reach the semifinals here.

The exits of Shriver and Mandlikova made it even more likely that Martina Navratilova and Chris Evert Lloyd will meet in the final. Each won today against a game opponent who gave the crowd a couple of thrills without ever really being a threat.

Evert, reaching the semifinals here for the 14th time in 14 tries, defeated Sylvia Hanika, 6-2, 6-3, this afternoon. Tonight, in a match that was more entertaining than it was close, Navratilova beat 6-foot-2 Helena Sukova, 6-3, 6-3.

The semifinal matchups are Evert-Bassett and, much to Shriver's horror, Navratilova-Turnbull.

Cash next will play Ivan Lendl, the No. 2 seed who was extremely impressive tonight in beating fifth-seeded Andres Gomez, 6-4, 6-4, 6-1. Lendl never lost his serve in a match full of wild shots that had the night crowd, which included Sen. Edward Kennedy (D-Mass.), screaming.

The yelling was as much to keep warm as in appreciation of the tennis. Temperatures dropped into the 50s.

The upset bug even carried over into the doubles, as three-time champions John McEnroe and Peter Fleming were beaten in a semifinal tonight by Anders Jarryd and Stefan Edberg of Sweden, 3-6, 7-6 (10-8), 7-5, 7-6 (7-2). That match had dual significance since it could be a preview of the Davis Cup doubles final. Both the United States and Sweden are still alive in semifinal play.

The match lasted 3 hours 13 minutes and McEnroe double-faulted on match point.

It was the second doubles match of the day for the losers, who earlier had eliminated John Lloyd and Dick Stockton, 6-1, 6-1, 6-3.

Cash's victory today, on an afternoon notable also for the swirling breezes that made play in the stadium extremely difficult, was not that big a surprise. Wilander still is not completely comfortable on this surface and, thanks to some faulty scheduling, had to play at 2 p.m. after finishing a rain-delayed match at 9 Tuesday night.

"That's not why I lost," Wilander said graciously. "I didn't play bad; Pat just played better. He was getting a lot of serves in and I was having trouble returning."

In fact, neither player broke serve until Cash slammed a backhand down the line in the 10th game of the second set for a clean winner and the set, 6-4. Having taken a two-set lead, Cash promptly disappeared into the cement for a bit, losing his serve three times in the third set.

But in the fourth, serving strongly again, he finally broke Wilander on his seventh break point of the set. Wilander slapped a backhand wide off Cash's forehand that was right on the line and trailed, 5-3. Cash served out the match at love.

It was the specter of Navratilova, winner of 53 straight matches, that seemed to hang over Shriver throughout her match with Turnbull. "I put a lot of pressure of myself because I so much wanted to get to the semis," said a very upset Shriver. "I just always seem to come up a match short at the Grand Slam events. Wendy's going to play Martina and I don't think that's good for women's tennis. I don't think it will be a very good match."

The record says that Turnbull is 4-25 lifetime against Navratilova, her last victory four years ago. Shriver beat Navratilova, her doubles partner, here two years ago and stretched her to a 7-5 third set several weeks ago.

"How many times has Pam beaten Martina recently?" Turnbull asked when told of Shriver's remarks. "She may think she has a better chance of beating her than I do, but right now, she has no chance of beating her."

Turnbull is one of those players who has hung around the top 10 for years but never been a threat to be No. 1. Recently, her ranking dropped to 14, the lowest in eight years.

Today though, even with a wrap on a sore hamstring, she was moving as her nickname -- "Rabbit" -- implies, seemingly getting to everything.

"Wendy served better than I have ever seen her serve," said Don Candy, Shriver's coach. "Wendy was up to play and Pam was not. Wendy hit 12 key passing shots and Pam didn't hit any, not one damn passing shot. Wendy was afire and Pam was smoldering."

The smoldering Shriver seemed to have the match under control until Turnbull broke her for the first time in the match early in the second set. "After that she got very nervous, she played one sloppy game and it got to her," Turnbull said. "I think she put a lot of pressure on herself."

The pressure seemed to get to Shriver in the last set. Quickly, she was broken twice by Turnbull and was down, 4-1. When Turnbull's net-cord ball crawled over the net for a winner in the next game, Shriver was furious.

"Why do you get all the lucky breaks?" she screamed at Turnbull. "Why do your net cords go over and mine don't? Why can't I get one break on an angle?"

Still muttering, Shriver stalked back to the base line. "It's not my fault," Turnbull said later. "I can't help it if God was on my side today."

Shriver, who double-faulted to end the match, was left to stew about misfortune and her inability to beat players she thinks she should beat.

"I told Wendy that I hope she plays well against Martina," Shriver said. "I mean, everyone's so sick of it -- not Martina, just all the questions about when someone's going to beat her. I really believe I can play with her but I have to start beating lesser players in these situations.

"I've beaten Wendy the last five times we played. If I didn't put so much pressure on myself Wendy wouldn't win five games against me. But that's my fault." Shriver paused and shook her curly hair sadly. "Shoot!" she said.

Mandlikova played one of those matches in which her brilliance and wildness were never more evident. Also evident was the gap between the top two women and the rest of the world. Many, including Mandlikova and Shriver, assert that Evert is now closer to them than Navratilova. Today, the gap appeared as wide as ever.

"There's a gap from 1 to 2 and another from 2 to 3, I think that's obvious," Evert said. "The balance in women's tennis is from No. 3 to No. 15."

Bassett must be credited for playing a strong, steady match today. Always a base liner in the past, she took the net at times, but mostly relied on superb passing shots to unravel Mandlikova. She said the mononucleosis forced her to work on her serve-and-volley game because, "My base line game was so bad when I came back." Later, she added, "I'm glad I had mononucleosis. I may try this every year.

"I knew Hana wasn't playing that well this week," she continued, her voice scratchy from laryngitis. "I just thought if I could keep myself from going too fast and not think too much out there, I could beat her."

Bassett began by ripping a backhand past Mandlikova for her first break of the match and took the first set with a forehand winner.

Mandlikova repeatedly tried to take the net even though she was having serious problems with her first serve.

Bassett quickly led, 3-0, in the second set, winning 10 straight points at one juncture, before Mandlikova turned around and won the next eight points to close to 3-2. But Bassett broke back in the next game when Mandlikova pushed a backhand wide and Bassett followed with a gorgeous backhand down the line.

"That game was real important because I wanted to break her right back and she was serving that game against the wind," Bassett said. "You know, on a given day Hana can beat Martina . . . Unless Martina is on a given day, too."

Today, though, was not one of those days as Mandlikova sprayed winners and errors all over the court as is her wont. Afterwards, she said she was tired the whole match and noted that Bassett played as if she had no pressure on her.

"She goes out there with nothing to lose because she has nothing to worry about," Mandlikova said. "If she loses, so what? Her father has millions of dollars, anyway, so it's no big deal." Admiringly, Mandlikova, whose father is a sportswriter, added, "She's a tough kid, though."

Bassett's father John, owner of the USFL's Tampa Bay Bandits, sat in the stands with New Jersey Generals owner Donald Trump. As he raced to greet his daughter, Bassett apologized for the suit he was wearing.

"I ran out of T-shirts," he said. "I never expected us to be in the tournament this long."