Late Saturday, at the end of a long day, John McEnroe walked off the court at the West Side Tennis Center and said, "Call Lendl and ask him who beat Vilas."

Ivan Lendl, who lost in the quarterfinals of the Mercedes Tournament of Champions, had predicted that McEnroe would never beat Guillermo Vilas, the clay-court specialist. Today, McEnroe defeated Vitas Gerulaitis, another player of reknown, 6-3, 7-5, to win the tournament, his first significant clay court title. He thanked his mother and everyone else "who made this win on clay more meaningful."

He declined to mention any names. But his meaning was clear. Anything else he wanted to say to Lendl? he was asked. "By winning I've said something," he said.

McEnroe, the serve-and-volley player, gave way to another McEnroe this week. For the first time in his career, McEnroe showed he has the patience and the shots to win on clay against those who know how to win on it.

He did it despite a sprained muscle in his right thigh; despite being fined $1,000 for calling Tomas Smid an impolite name and a Communist during their quarterfinal match. The only thing he would have done differently this week was, "I probably would have waited till the locker room to say it and saved myself $1,000."

Unquestionably, this is the best he has ever played on a slow clay-like surface (Har-Tru). But, as he said, "It's not the hardest thing to compare. The bottom line is I haven't won any tournaments (on clay). I've had good results. I won Boca West but that was only four players and two wins. And I've won one other clay court event, a small tournament (South Orange, 1979, against John Lloyd). So I've basically won nothing. It wasn't like I beat Borg in the semis and Connors in the quarters."

Obviously, it slipped his mind that he won the mixed doubles at the French Open in 1977 with Mary Carillo, when he was 18. Now, at 24, he is leaning toward playing singles at the French, beginning May 23, a grinding test of his clay-court resolve.

But first, he said, "I want to make sure I have the doctor's okay and that I can get in one week of hard work."

Part of the satisfaction for McEnroe this week was due to the fact that he was able to make the transition from an indoor surface, and indoor game, to an outdoor surface and an outdoor game. Last week, he beat Lendl in the finals of the WCT championship in Dallas, no small feat.

Today, he beat Gerulaitis for the first time on clay. The last time they met in the final of this tournament three years ago, Gerulaitis won. Today, Gerulaitis took the initiative. He pushed and pressed and came in whenever he could; more than McEnroe expected. Gerulaitis forced the pace from the beginning, knowing that if he didn't, McEnroe would.

McEnroe retaliated with patience and lobs so perfectly placed he couldn't have been happier if he had deposited them by hand. McEnroe's offensive lobs are often superb; today, he said, they were better than normal. At least five went for outright winners (several others, Gerulaitis mis-hit). One came at 2-1 in the second set, break point against McEnroe, the only break point Gerulaitis had in the set. It was a backhand lob that caught Gerulaitis flat-footed at the net. There was no point in chasing it.

When he wasn't lobbing to perfection, McEnroe was using his drop volley.

Early in the match, McEnroe's serve was shaky, as it was against Vilas in the semifinal. He missed seven of eight in the first game, four of six in the third (35 percent for the set).

Though they made almost an equal number of unforced errors, Gerulaitis always seemed to make them at a critical moment. He double-faulted to give McEnroe the first break point of the match and volleyed a backhand long to give him a break at 3-1. Gerulaitis had a chance to break back but McEnroe saved two break points with his serve, which surrendered to his control when he needed it most.

McEnroe broke again to make it 5-1 and it looked like Gerulaitis might just kiss the set goodbye. Instead, he toughened, the tempo increased; the level of play improved. Gerulaitis won two games; it became a match.

The second set was close, very close. McEnroe said he was lucky to win it. Each had only one break point until the 11th game. Gerulaitis fell behind 0-30 in the ninth game but stiffened and held.

The sun went down, the air got colder. McEnroe served and went up 40-15. Gerulaitis fought back to deuce; his best and last chance. McEnroe missed another first serve; Gerulaitis came in, attacking the second one. McEnroe hit a perfect forehand passing shot in the corner. Serving and volleying, for a change, he won the game and it was 5-all.

Gerulaitis fell behind, 0-30, then 15-40. He won the next point, a temporary reprieve, with quick, reflex vollies at the net. On the second break point, he came in again, hit a backhand volley. This time, he was was passed with a forehand down the line.

McEnroe served for the match at 6-5. Once again, as he had for the first set, he served an ace and a service winner. At 30-0, he lofted a final, offensive lob over Gerulaitis' head. Gerulaitis backpedaled, waving futilely at the sky. One point later, McEnroe hit a backhand that Gerulaitis swore was wide. He played it and stood at the sideline where he thought it had landed. McEnroe hit a stretching backhand volley into an open court, where Gerulaitis should have been.

McEnroe went home with $100,000, and $9,500 in bonus money, plus $150,000 last week in Dallas, feeling very nicely about the last eight days. "I feel good," he said. "But I think Barclay's Bank isn't too happy."