The curtain finally comes down on the 1977 tennis season Sunday, eight days into the new year, with the same players who were preeminent when the overture was played: Jimmy Connors vs. Bjorn Borg in the title match of the $400,000 Colgate Masters Tournament at Madison Square Garden.

But the top billing has changed in 12 months. There no longer can be any reasonable doubt that Borg is the No. 1 man for this season. The 21-year-old Wimbledon champion clinched that mythical honor today by beating Guillermo Vilas, regardless of what happens against Connors in the nationally televised finale for a $100,000 first prize (WTOP-TV-9, 4 p.m.).

Vilas, the 25-year-old Argentinian lefthander who had won 80 of 81 tournament matches in the last six months, decided his only chance against longtime nemesis Borg was to attack at every opportunity, but Borg's extraordinary quickness and passing shots ruined his designs on commanding the net.

Borg, content to keep the ball in play with his topspin ground strokes as long as Vilas cared to rally, made his biggest shot as counterpunches and won decisively, 6-3, 6-3, in 1 hour 32 minutes.

Connors, who lost a pulsating three-hour epic to Vilas Thursday night in the round-robin portion of this final playoff for the top eight finishers in the 1977 Colgate Grand Prix point standings, needed all his resources to subdue Brian Gottfried, 6-4, 3-6, 6-3, in the other semifinal.

Connors, the dominant player of the last four years, but only No. 3 for 1977 after finishing as runner-up to Borg at Wimbledon and to Vilas in the U.S. Open - won five points in a row from 0-40 on his serve at 1-1 in the final set.

That lost opportunity haunted Gottfried. The aggressive Floridian continued to battle, volleying superbly and scrambling for some remarkable "gets." He even knocked over a sidewall desperately lunging at Connors' last grunting overhead smash, trying vainly to save a third match point.

But Connors is still a fraction tougher than his fellow 25-year-old American. The crowd of 12,600 - which caused frequent interruptions with shouts, most of them pre- or anti-Connors - acknowledged that as they shouted appreciatively at the end of the entertaining 2-hour 18-minute match.

Only about half the spectators arrived in time for the 11 a.m. start of the Borg-Vilas match.

Vilas, restricted only slightly if at all by the strained tendon in his left foot that caused him to default his last round-robin match Friday, also seemed to arrive at the Garden it bit late.

He started slowly, double-faulting three times - twice on break point - to fall behind, 0-3. Even though he got one of those breaks back in the fourth game, he never climbed back into the first set.

Borg, showing no aftereffects of the flu that prompted him to default his Friday round-robbin match to Gottfried, broke for 3-1 in the second set on his fourth advantage point.

He cranked a forehand crosscourt pass after Vilas approached on a short ball, then broke with a running forehand down the line into the corner.

Vilas had shoved Borg out of position with a hard, deep ground stroke, come to the net behind a good backhand, hit a forehand volley down the line, and gotten passed. In short, he controlled the point, and Borg won it.

That happened frequently. Borg's speed made the court too small for Vilas. His ability to make reasonable shots from defensive positions while sprinting from side to side and his knack of threading the needle on passing shots blunted the effectiveness of Vilas' attack.

"Vilas cannot do it if he has to hit five winners to make the one point," observed one Italian journalist at courtside. "And if there is a tiny hole, Borg finds it." He was dead right, on both counts.

Vilas roused the crowd with one last stand, holding after seven deuces and three break points as he served at 1-4 in the second set. Twice he saved himself with slashing volleys that clipped the net and plopped over.

He broke back to 3-4 in the next game, as Borg double-faulted to break point and raked a backhand into the net.

But Borg broke again for 5-3, as Vilas sailed a backhand long for 30-40 and the let a blooping backhand return of a good serve go. It fell just inside the baseline for a winner. Borg served out the match at love.

That was the kind of discouraging day it was for Vilas, who is 4-12 in career meetings with Borg and 1-8 since 1974.

The victory gave Borg the nod over Vilas - French and Forest Hills champion - for the No. 1 ranking. Despite Vilas' protestations that the year ended Dec. 31, the Masters is the last event of 1977, much as next Sunday's Super Bowl climaxes the 1977 football season. And Borg's Wimbledon triumph, 56-2 tournament record since April, and 3-0 record over Vilas make him the clear-cut player-of-the-year.

Connors' opinion on the ranking situation was simple: "You guys (the press) rank us. I just go out and play tennis."

He played exceptionally well today in thwarting Gottfried's determined effort and showed why he is a champion in escaping from that 0-40 hole in the third game of the final set. Gottfried overhit a forehand volley, his favorite shot, off a blistering Connors drive at deuce and the couldn't make a forehand return off a big serve.

Connors went on to break for 3-1, after Gottfried had saved himself with a couple of spectacular low volleys, and served out the match.