U.S. divers took advantage of a big belly-flop to win silver and bronze medals in women's platform diving Sunday. America's swimmers and gymnasts just flopped as competition got serious at the Seoul Olympics.

The Soviet Union won the first gold medal of the Games, in women's air rifle, and the U.S. basketball, volleyball and boxing teams got off to successful starts.The basketball team, in fact, may be even better than when it won the 1984 Olympic final, beating Spain by 31 points. On Sunday, the Americans beat virtually the same team by 44 points.

Xu Yanmei of China won the diving with 26-year-old Michele Mitchell of Boca Raton, Fla., taking the silver in the final meet of her career, and Wendy Williams of Bridgeton, Mo., capturing the bronze.

Chen Xiaodan, a 14-year-old from China, led until she nearly belly-flopped her last dive, a difficult backward 31/2-sommersault, and she finished out of the medals. That also opened the door for Williams to finish third.

"I think today was the experience of age vs. the resiliency of youth, and I was hoping that experience would pay off," Mitchell said. "And it did for me."

The swimmers, though, particularly the women, were a disappointment in Sunday's preliminaries, as were the men's gymnasts.

Both Matt Biondi and Janet Evans, America's two best swimmers, won their individual heats but finished behind Eastern Europeans.

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Not to worry, said Coach Richard Quick.

"They did what they intended to do, which was to get to the finals in good position," Quick said. "It looked like Matt didn't want to push it all the way. He knew he was in good shape."

Evans, of Placentia, Calif., favored to win three swimming gold medals, won her qualifying heat for the women's 400-meter individual medley, but she had only the third fastest time of the day.

Noemi Lung of Romania was fastest in 4 minutes, 41.96 seconds, while Evans had 4:43.04. Erika Hansen of King of Prussia, Pa., was 11th, failing to qualify for the final by three places.

Biondi, of Moraga, Calif., who is entered in seven events, could do no better than second-fastest in the men's 200 freestyle. Although he won his heat and had a faster time than archrival Michael Gross of West Germany, Biondi was second-fastest to Artur Wojdat of Poland.

Wojdat was timed in 1:48.02, Biondi in 1:48.39.

And those were the least of the swim team's problems.

Top-ranked American Mitzi Kremer of Titusville, Fla., failed to make the final of the 100 freestyle, while Dara Torres of Beverly Hills, Calif., swimming because Angel Myers was banned for testing positive for steroids, barely made it by finishing seventh.

Rich Schroder of Lindsay, Calif., ranked second in the world, was only the seventh qualifier for the 100 breaststroke, and Daniel Watters of Pensacola, Fla., failed to advance.

In the first day of gymnastics, the U.S. men's team finished three points behind Italy in the compulsories, with the powerful Soviets and most other teams yet to begin competition.

"I can think of at least five teams that will beat us," said Peter Vidmar, a silver medalist in all-around four years ago. "It will be very difficult for us to even make the top six after today."

In its first basketball game of the Olympics, the U.S. team got 16 points from David Robinson for a 97-53 victory over Spain, which has six players returning from its 1984 silver medal team. Robinson, in the Navy since graduating from Annapolis a year ago, led five U.S. players in double figures.

Hersey Hawkins and Danny Manning had 13 points apiece, and Dan Majerle and Charles Smith IV each had 12.

In volleyball, the United States extended its winning streak over Japan to 28 matches.

"Streaks are made to be broken," U.S. team captain Karch Kiraly said, "but we didn't let it happen today."

Japan led the favored United States, playing without setter Jeff Stork, 10-6 in the first game before the U.S. team rallied to a 15-13, 15-2, 15-2 victory.

The winning shot came on a kill by Steve Timmons, one of the most feared spikers in the game.

In boxing, the U.S. team made a successful debut when Arthur Johnson of Minneapolis won his bout with tough Andrea Mannai of Italy in the 112-pound class.

"I wanted to get the first win for the team," Johnson said.

Irina Chilova of the Soviet Union won the women's air rifle with 498.5 points after 10 shots, followed by Silvia Sperber of West Germany with 497.5 and another Soviet, Anna Maloukhina, at 495.8. Launi Meili of Colorado Springs, Colo., was sixth with 493.3 points.

In men's 100-kilometer team time-trial cycling, East Germany won the gold medal, followed by Poland and Sweden.

The world's two fastest humans, Ben Johnson and Florence Griffith Joyner, meanwhile, proclaimed themselves ready to race.

Johnson, of Canada, world record-holder in the men's 100 meters, and American Florence Griffith Joyner, who holds the world record in the women's 100, both said they were untroubled by injury. Johnson injured a hamstring during the indoor season, and Griffith Joyner was rumored also to have a hamstring injury.

"I'm ready to go," Johnson said. "I haven't felt this good since Rome." It was in Rome at the 1987 World Championships that Johnson set his world mark of 9.83.

Griffith Joyner, meanwhile, denied rumors that she had a pulled leg muscle, although sprint coach Russ Rogers has said she suffers from some minor soreness. Rogers said he didn't expect the soreness to affect her race, though.

"My main competition is myself," she said. "If I concentrate on what I have to do, I will come out a winner."

Griffith Joyner, who set her world mark of 10.49 at the U.S. trials in July, plans to compete in the 100, 200 and 400 relay.

This bustling Asian metropolis of 10 million came to a virtual standstill as the Games were declared officially under way at Saturday's opening ceremonies.