The eagerly awaited Great Showdown between Jimmy Connors and John McEnroe turned into the Great Slowdown and then the Great Disappointing Thud tonight as Connors, trailing by 5-7, 0-3, defaulted from the $400,000 Grand Prix Masters Tournament with a blister on his left foot.

Ironically, under rules tightened because Bjorn Borg and Guillermo Vilas defaulted their final round-robin matches last year after having already qualified for the semifinals, Connors was disqualified from further competition in the Masters enen though he said he would have tried to play his final round-robin match Friday.

Connors' forced departure further discolored a tournament already tarnished by the failure of Borg and Vilas to accept invitations, even though they finished among the top eight in point standings for the season-long Colgate Grand Prix of tennis.

The Masters is supposed to be a showcase playoff for the top performers of the season just concluded, a kind of Super Bowl of tennis. But since its inception in 1970, it has been plagued by no-shows, injuries and bizarre and controversial incidents.

The tournament uses an unusual format in which the eight players are divided into two groups for three days of round-robin play. Then the two players from each group with the best round-robin records avance to the semifinals, the leader of each group playing the No. 2 man of the other.

Last year, Vilas and Borg were 2-0 after two days and guaranteed of places in the semifinals, so they took advantage of a loophole in the rules with "strategic" defaults. Vilas skipped his last match to rest a sprained ankle and Borg followed suit, claiming a fever and flue symptom.

There was a great outcry from customers who already had bought tickets, and further objections from people who pointed out that Borg had been able to select his semifinal opponent. By defaulting his last match, he assured himself of finishing second in his group and playing Vilas, his "pigeon," instead of Connors, his nemesis.

This year the loophole was closed, but defending champion Connors -- he beat Borg in the final last January -- wound up getting caught in it.

This was doubly ironic because Connors did not intend to play the Masters until he was talked into it last week, and his prospective showdown with McEnroe, the ascending 19-year-old left-hander who led the U.S. victory in the Davis Cup final last month, was hailed as the tournament's saving grace.

With Connors out, Arthur Ashe -- who lost to McEnroe Wednesday -- gets a default over Connors in their scheduled Friday match. McEnroe (2-0) and Ashe (2-1) thus have qualified for the semifinals, with McEnroe No. 1 because of his head-to-head triumph over Ashe.

In other matches today, Brian Gottfried defeated Corrado Barazzutti, 7-6, 6-4; Ashe handed Harold Solomon his second defeat, 6-1, 6-4, and Eddie Dibbs thrashed a barely resisting Paul Ramirez, 6-0, 6-1.

Gottfried and Dibbs thus improved their round-robin records to 2-0 and qualifkied for the seminfinals. Solmon, Barazzuti and Ramirez are 0-2 and out. The only question remaining Friday is the pairings: McEnroe will play the loser of the Dibbs-Gottfried match. Ashe will play the winner, when Saturday's anticlimactic semifinals finally roll around.

The 16,100 customers who came to Madison Square Garden Xpecting to see a shootout between King Connors and Prince McEnroe, the heir-apparent as brash and talented as the U.S. Open champion, had no idea that Connors had been nursing a deep blister for "four or five days." It had not been apparent as Connors bludgeoned Solomon in his opening match on Wednesday.

But as he served at 4-5 in a close, tense first set -- McEnroe had escaped two break points in the first game and another in the seventh with some clutch serving -- Connors started limping.

His discomfort became more obvious in successive games, especially after McEnroe broke his serve for the first set on his first break point of the match.

Connors stopped sprinting for some shots, and started making an uncharacteristic number of unforced errors as McEnroe cleverly moved the ball around, changing pace and spin. On the final two points of the set, Connors missed passing shots by inches after McEnroe chipped superb approach shots and darted to the net. After the first game of the second set, Connors took a three-minstered to jury timeout" and was ministered to by trainer Todd Fullerton and tournament physician Norman Rudy.

MeEnroe broke serve in the second game after four deuces, Connors badly netting two forehands off god approaches, then watching forlornly as McEnroe blasted a forehand down-the-line approach and raced in for a backhand volley winner.

After one more game, which McEnroe held at love, Connors quit.

Dr. Rudy said Connors had a hemorrhage under a combination of blister and callous between the big toe and arch of his left foot. "It's a significant blister, bery deep, very painful," he said, adding that he had removed blood from the wound Wednesday and cushioned it with a protective padding before tonight's match.

MeEnroe, who served exceptionally well on critical points despite getting himself in trouble with seven double faults, raised his arms in triumph after the 1-hour 16-minute "technical knockout," obviously satisfied despite the hollow nature of his triumph.