Paul McNamee, an Australian who almost quit the tennis tour in August to go back to law school, let Guillermo Vilas escape six match points tonight before finally upsetting the fourth-ranked player in the world, 4-6, 7-5, 7-5, 2-6, 6-3, in a WCT record 4-hour 32-minute final of the Maryland Tennis Classic at the Towson Center.

McNamee, ranked 59th on the computer coming in, earned $100,000 for the victory attended by 3,500. His prior 1982 winnings were $40,500. Vilas' second place here paid $40,000.

Vilas, who was eliminated in the first round in three of his previous five tournaments, seemed in control after winning the fourth set to tie the match and breaking McNamee in the first game of the final set at love. But McNamee, content to hammer with Vilas from the base line after being restless for four sets, won the next four games and held on for the set.

McNamee, 27, the 1982 Wimbledon doubles champion with partner Peter McNamara, had lost to Vilas the previous three times they met by a combined margin of 48 games to 18. This was his third singles championship.

McNamee came out aggressively, charging the net to take a 3-0 lead in the first set, winning a break point as Vilas twice double-faulted in the second game. But Vilas then reset the pace, quickly abandoning his powerful serve and opting for a slower, more accurate high-bouncing serve. The Argentinian won nine of the next 10 games to take the first set and move ahead, 3-0, in the second set, only to see McNamee fight back to win it.

Trailing 2-1 in sets that each took at least an hour, Vilas again changed rhythm. He stayed behind his base line, but played deep to McNamara's base line rather than alternate shots to the sidelines, as he had done earlier.

Still nailed to the base line by Vilas in the final set, McNamee changed his fate by eliminating the unforced errors that plagued him throughout the initial four sets.

On a free Sunday afternoon, Coach Frank Kush of the Baltimore Colts was on hand to spin the racket that decided the choice of first serve.