Pardon me, sir, but didn't you used to be Arthur Ashe?

That was the question running through the minds of 12,200 enthralled, enthusiastic onlookers at Madison Square Garden today. They watched as a skinny, sublimely cool man with electricity in his serve and greased lightning for a backhand came flashing out of the past, restoring interest in the $400,000 Colgate Grand Prix Masters tennis tournament.

Yes, it was Ashe -- not quite the player who was ranked No. 1 in the world in 1975, but a reasonable facsimile -- who beat Brian Gottfried, 7-5, 3-6, 6-3, to reach his biggest final since he won at Wimbledon 3 1/2 years ago.

Ashe -- playing craftily, intimidating Gottfried with his whiplike returns of serve -- advanced to Sunday's rich title showdown with young hotshot John McEnroe (WDVM-TV-9) at 4 p.m.

This return bout -- McEnroe clobbered a not-yet-plugged-in Ashe, 6-3, 6-1, in the round-robin portion of the Masters Wednesday -- is sure to refocus attention on this eight-man playoff for the top finishers in the 1978 Colgate Grand Prix of tennis. Earlier, the tournament seemed irreparably diminished by the truancy of Bjorn Borg and Guillermo Vilas and the default Thursday of defending champion Jimmy Connors against McEnroe.

McEnroe, whom Ashe calls "unquestionably the best player in the world the last four months," toyed with Eddie Dibbs, 6-1, 6-4, in today's other semifinal. He has won 48 of 55 matches since losing to Connors in the U.S. Open semifinals last September.

Sunday's winner will collect $100,000, the runner-up, $64,000.

Win or lose, the check will be the biggest McEnroe has seen since he dropped out of Stanford and turned pro in June.

More surprisingly, it will also be the biggest payday for Ashe, who first came to New York to play in the U.S. Open at Forest Hills in 1959. McEnroe was then 8 months old.

The finale figures to be much closer than 55-minute hatchet job McEnroe inflicted on Ashe Wednesday. Ashe never got started in that match. He never got into the groove of returning McEnroe's wicked left-handed serve, which skids with snake-like venom off the quick, low-bounding synthetic carpet here.

"I think I might be there this time, mentally and physically... The last time, my mind was somewhere else," said Ashe, who prepared for his first match by attending two marathon days of meetings of the Men's International Professional Tennis Council. An elder stateman at age 35, he is one of three player representatives on the council, which governs the Grand Prix circuit.

"It was very difficult for me, but I had to be there because we were taking crucial votes," added Ashe, who is lucky to be in the Masters at all, to say nothing of the final.

He finished 10th in the season-long Grand Prix standings. He went to Australia to play two tournaments last month in a last-ditch effort to crack the top eight, but was thwarted when John Marks beat him in five sets in the semifinals of the Australian Open. Ashe got into the Masters only because Borg and Vilas declined invitations.

After being blown out by McEnroe on Wednesday, he beat Harold Solomon and then got a victory by defeault over Connors (blood blister on his left foot) to finish 2-1 in the round-robin and gain a berth in the semis.

"Maybe the way I played today justified my getting in the back door," Ashe said.

The crowd loved the way Ashe played today. It reveled in the discovery that the Splendid Splinter of tennis can still slice a magnificent serve when he needs one, and crackle a backhand.

Ashe has recovered from surgery on his left heel that sidelined him in 1977 and nearly ended a career that had brought him the U.S. Open (1968), Australian (1970), World Championship of Tennis (1975) and Wimbledon (1975) titles. He is No. 13 in the computerized world rankings of the Association of Tennis Professionals. A year ago he was No. 257 when he began his comeback, after nearly 11 months on the shelf.

"I was really eager to play again. I was happy to practice with anybody, to just go out and hit balls... I loved being down 3-5, 30-40, with one loved being down 3-5, 30-40, with one serve to come. I just loved being around the locker room, being back in the action again," Ashe reminisced.

"My goal was to be back among the top 15 players on the computer by the end of the year, and I made that. Everything else is gravy. But when you get back toward where you were, there's also pressure. You have a reputation, a position, to protect again."

Ashe served patchily, missed some easy volleys, and overanxiously hurried into some unforced errors today. But when he had to, he protected his reputation splendidly.

When he lost his serve in the fourth game of the match, delivering the first of four double-faults and then bungling a backhand volley, he broke in the next game, preventing Gottfried from getting on top.

Ashe saved three break points and held after four deuces in the eighth game, producing four good serves after a horrible stretch in which he missed 13 of 16 first balls. Then he won the set after serving for it at 5-4, and being broken again.

Ashe let Gottfried off the hook after four break points and six deuces in the seventh game of the second set, then lost his serve at 15 after double-faulting twice to 0-40. The set vanished, and Gottfried broke again in the first game of the third as Ashe dropped four games in a row and missed 11 of 13 first serves.

But King Arthur came back, led 3-1, and came back again after losing his serve in the fifth game. He was toughest when the points were most critical, gambling and winning.

"Brian doesn't like to play me on this surface because I intimidate him with my return of serve. He knows I'm not going to play safe. If he misses his first serve, I've going to belt the second and come in," said Ashe, who cracked some vintage backhands.

Gottfried -- who had beaten Ashe in four of their last five meetings, but lost to him in a final at Los Angeles in September -- acknowledged that he was intimidated.

"That's pretty true. He returned awfully well, especially when I was ahead." Gottfried said.

"They weren't any little chips by my feet. They were 400-mile-an-hour screamers at my feet.

"That works to his advantage two ways. He won a lot of points with returns, and he also made me try to serve better and better. Instead, I served worse and worse."

Like McEnroe, a young master of spin and change of pace, Ashe cleverly varied speeds during rallies, beating Gottfried as much with cleverness as the perfectly timed power that used to be his calling card.

"He played an awfully smart match," said Gottfried. "I think Arthur's playing as well as ever."

McEnroe had an easy time against Dibbs, whose only hope was to return serve well. He didn't, and that was that.

McEnroe, though suffering from a cold and nosebleeds, raced to a 5-0 lead in the first set, and never lost his serve. He outrallied Dibbs from the back court and carved him up at the net, darting in behind chipped approach shots and forcing returns of every second serve.

"He's quick, he's got a good serve, he's got great touch. Since the U.S. Open, McEnroe has been the best in the world," Ashe said of his opponent in the final. "But like everybody else, he can be had... I can play with anybody on this surface."

In the men's doubles final, McEnroe and Peter Fleming defeated Wojtak Fibak and Tom Okker, 6-4, 6-2, 6-4.