The San Antonio Spurs blew an 18-point second quarter lead, but regrouped in time to down the Philadelphia 76ers, 111-108, tonight in the deciding seventh game of their NBA Eastern Conference semifinal playoff series.

In winning their first seven-game series after losing nine of them in a row, the high-scoring Spurs advance to the Conference final series against the Washington Bullets beginning Friday at Capital Centre.

George Gervin was magnificent, leading the Spurs with 33 points, 12 rebounds and five assists and Larry Kenon turned in a strong game as well with 27 points, but it was stringbean reserve center Mike Green, the man his teammates call Snake, who was the difference.

Playing 39 minutes because regular center Billy Paultz was on the bench with a pulled hamstring muscle, Green scored 20 points and added eight rebounds, three steals, three assists and four blocked shots.

The Spurs were nursing a one-point lead when Green made two free throws after being fouled by Bobby Jones with 1:04 to play. After Julius Erving got the 76ers back to within a point with two free throws, Green scored on an 18-foot jumper to get the San Antonio lead up to 109-106 with 43 seconds to play.

Bobby Jones missed at the other end for Philadelphia and Green rebounded. Gervin then made a pair of free throws with 11 seconds left for a five-point lead and the 16,055 fans at HemisFair Arena went delirious.

Maurice Cheeks scored as the game ended to account for the final margin.

"There was a little pressure on me, but I can handle it," the 6-10, 200-pound Green said. "There was a lot of criticism about our centers lately and I didn't think it was right."

The Spurs came out firing and Gervin scored 22 of his points in the first half. He scored 12 in a row in one stretch of the first period when the Spurs first took the lead. With the 6-7 Iceman passing for assists, rebounding and diving on the floor after loose balls, the Spurs were soon ahead, 43-25.

The 76ers stayed with their game plan, however, which was to get it to the Doctor. Erving reponded with 34 points, eight assists, three blocked shots and teamed with Maurice Cheeks and Steve Mix to get his team into the game.

Those three scoed 25 third-quarter points as the 76ers outscored the Spurs, 34-22, to take an 81-78 lead into the final 12 minutes.

"We always play a good fourth quarter so I wasn't worried," Green said.

The Spurs finally regained the lead on a three-poing play by Mark Olberding with 8:52 to play and the lead changed sides six times before a Gervin free throw gave the Spurs a 105-104 lead with 1:27 left.

Cheeks shot an air ball from 22 feet on Philadelphia's next possession and the Spurs held on.

"Sometimes it's tougher to hold on to a lead than it is to catch up," said San Antonio Coach Doug Moe.

"I felt we would come back even after we lost that big lead," said Gervin. "The thing we had to do was keep our confidence and we did.

"I had a job to do and I came out to do it from the very beginning. They said Maurice Cheeks was holding me down. He's good, but he's still a rookie and he has a lot to learn."

The 76ers battled back from a 3-1 deficit in this series to tie it at 3-3 after switching Erving to gurard, Caldwell Jones to forward and putting Darryl Dawkins in at center. Dawkins sprained his ankle in the first period tonight, however, and ended the game with only three rebounds and 11 points.

With Dawkins not much of a factor, the Spurs outrebounded the 76ers 45-36 and got five more offensive rebounds. CAPTION: Picture, Far Out of Reach George Gervin of Spurs dribbles in effort to maneuver around 76er guard Henry Bibby in San Antonio victory. AP