Bill Cosby attended the U.S. Open, but he didn't want to wait around for the tennis matches. Diane Sawyer had come to watch, Tom Brokaw had come to watch, Tatum O'Neal had come to watch, but the biggest television star in America had come to play.

Not to win, exactly, and really not so much to play, either, as to play around, which is what Cosby did yesterday morning a few hours before the day's actual matches were to begin in the National Tennis Center. Cosby galumphed cheerfully onto the courts as part of a celebrity doubles exhibition sponsored by Sony.

Top-rated (NBC's blockbuster hit "The Cosby Show") if not precisely top-seeded, Cosby at least walked away with sportsmanship honors, implicitly anyway. At one point he and his partner Vitas Gerulaitis were having so much fun they just batted the ball between them, ignoring Virginia Wade and Peter Fleming on the other side of the net. Much of Cosby's time on the court was spent battling not his opponent but the referee, none other than he who there truly is none other than, Howard Cosell.

"Cos, this is not the time for truculence. It is the time for jocularity," Cosell lectured from on high, in the referee's chair. Play resumed, with Cosby alternately clowning gracefully and backhanding gracefully, while kids wearing "I Love Sony" T-shirts got it all on tape.

When he flubbed a particularly easy shot, Cosby said, "The cameras weren't rolling, were they?" After completing a particularly sensitive, even genteel, serve, Cosby declared grandly, "And now for the return!" After completing a sizable burp, Cosby said, "We can edit that out." Soon he was calling out, "How-word! How many seconds left?"

Cosell was taciturn if not obdurate. Cosby was resplendent, after a fashion, in what was not so much a tennis outfit as a global tour and curriculum vitae. His T-shirt was imprinted "Hotel Du Cap, Eden Roc, Cap D'Antibes." A red baseball hat worn atop a sweatband carried a big "T" for Temple University. Over this he later put his University of Pennsylvania varsity tennis blue-and-red jump suit. This was for the most exhausting exercise of the morning, signing autographs at courtside.

"Don't get excited! Don't get excited!" Cosby kept telling the small mob of young fans who surged forward, but how could they not get excited? Cosby's not just a gigantic star, he has more charisma than Santa Claus. When a plump kid handed him a Coke (Cosby does Coke commercials on TV), Cosby exclaimed, "Fat Albert! How you doin'?" and poured the Coke into a paper cup for him.

Meanwhile Gerulaitis had grabbed one of the Sony Video 8s and was recording the ritual for posterity, or rather to be played back 20 minutes later in the Sony hospitality suite. Cosby turned to the camera and facetiously captioned the scene: "Signing for the poor people," he said. Back on the court, ABC News anchor Kathleen Sullivan was batting around a few with Fleming. She had teamed with Cosby for an earlier match against "Good Morning America" travel editor Stephen Birnbaum and Virginia Wade. Asked about the games, Cosby told one fan, "Hey, once we got rid of Kathleen we were all right." Sullivan said later, "We never even stepped on the court. I mean, this was the WORST tennis!"

Cosby wasn't just signing autographs on paper now, he was signing people -- the shirts on kids' backs, at their request. At this point the best laid plans of Sony went slightly awry. Cosby and Cosell and the tennis players were to go to the hospitality suite and watch the tapes played back. But Cosell grabbed Cosby and suggested they get a drink in the U.S. Open club. So they did, dragging Sullivan and a deeply amused reporter with them.

While Sony representatives searched for Cosby and Cosell, they were tucked away at a little white table doing a very funny Bill-and-Howard show.

"Bill Cosby, Phylicia Ayers-Allen made you the star you are today!" Cosell bellowed at one point, referring to the talented actress who play's Cosby's wife on his show. "Without her you are nothing!" Cosby just smiled. "Ah yes, it's her bottom lip," he said. "She just sticks out her bottom lip and we get a 41 share."

Then he reached over and grabbed a handful of Howard Cosell's rather bounteous paunch. Cosell was chagrined. He claimed to be pregnant. "I've asked you time and time again," Cosell started to say, but Cosby still had hold of him. "There is no child," he said with finality. "It's bloat."

Henceforth, Cosell rooted about for contentious remarks to make. He told Fleming, who had happened by, that his body was only a dilapidated shadow of its former self. He got into a dispute with Cosby over whether northern Philadelphia was predominantly black or predominantly Italian when Cosby was growing up in Philly some years ago. Then it was time for Sullivan, surrounded by her luggage, to be accused by Cosby of having stolen the bathrobe from the last hotel she visited.

Enter Martina Navratilova, wearing a "Ghostbusters" T-shirt and still unhappy about having lost the women's finals to Hana Mandlikova on Saturday. There were condolences all around. "I was watching TV last night and someone said this was great for women's tennis," Navratilova said. "Click! Hah! My losing helps women's tennis!" Cosby stood up and smiled at her. "Martina, let me tell you something," he said, taking her aside for a muttered tete' a tete', which clearly had the effect of cheering her up.

While he was thus occupied, Howard Cosell said of Bill Cosby, "The best man in show business, bar none. Always was. Always will be. We go way back, you know. Way back."

A Sony representative appeared to inform the two men that they were supposed to be elsewhere. The party began to split up. Cosell was preceded through the halls and banquet rooms by his cigar, but not by Cosby, because at almost every step he took, someone appeared with a request for an autograph. He had his picture taken with busboys and with ball boys, and everyone who spotted him looked thrilled and delighted in a way that probably no tennis star on Earth could have thrilled and delighted them. It was all pretty thrilling and delightful.

Cosby was further besieged when he got to Sony's room, where the videotapes were being played back on monitors. Cosby glanced at them but was not terribly interested in seeing himself on TV just then. Before leaving he confided that he is working with Lena Horne, who made a guest appearance on "Cosby" last season, on a series of her own, on which she would play a hip grandmother who wants to help her daughter raise her children. But as the time for the real U.S. Open tennis matches approached, Cosby said he wanted to leave so he could go see them -- on television. He declined an offer to sit in the Sony court-side box and watch them in person with Sony Chairman and founder Akio Morita, who had flown in from Tokyo the night before.

"I like to watch it on TV, so that if something happens, you get to see it eight or nine times," Cosby said. "When you're sitting in the crowd, and you turn around to talk to somebody, suddenly you hear a big roar and something's happened and you've missed it. When I was 12 years old, I was on my way to a baseball game with some friends, and we were on the subway when we heard it was going to be televised. And I got off the subway and went home and watched it on TV."

He and Cosell, one felt, Would Meet Again. "You're a lost man, Howard, which is worse than being marked," Cosby had told Cosell, to which Cosell audaciously replied, "Cos, you're a tough little monkey," causing Cos to slap his forehead in exasperation and say, "Oh no! Now I'll have to defend you all over again!"

Then Cosby had gone into an impromptu monologue about John McEnroe's temperamental nature, as evidenced Saturday when, during the men's semifinals, in 105-degree heat, McEnroe had demanded that a woman fanning herself in one of the upper boxes stop it before he would continue playing.

"I love it," Cosby said. "McEnroe says, 'Would you tell the lady to stop keeping herself from passing out?' " He imitated McEnroe, demonstrating how he could not possibly see the woman fanning herself and keep his eye on the ball as he served. Sullivan was in stitches, Fleming was in stitches, even old Howard Cosell dropped a stitch or two. "This is in his next show!" Cosell insisted. "I tell you to God, this is in his next show!" But no, it was a private performance, and a privilege to witness.