A free throw by Norm Nixon with 85 seconds left tonight provided the winning point and the Los Angeles Lakers survived a last-second missed shot by Mike Mitchell to beat the San Antonio Spurs, 101-100, and advance to the NBA championship series against the Philadelphia 76ers.

Mitchell's shot was partially blocked by the Lakers' Magic Johnson, who got a hand on the ball. "I wasn't worried," Johnson said. "When they had the ball the last time I knew I was just going to run and double-team them and make them take a shot they didn't want to take. My instincts just took over."

So did Los Angeles, winning three times on San Antonio's home floor to capture the Western Conference championship, four games to two, and for the third time in four years gain the NBA finals. The final series opens Sunday afternoon in Philadelphia.

Los Angeles beat Philadelphia in the finals in 1980 and 1982.

Nixon, who halted a San Antonio surge just inside the five-minute mark with a steal and basket, got one of two free throws that gave the Lakers a 101-96 advantage. But from that point on, the Spurs rallied. Artis Gilmore brought them within three with 98 seconds left and, after a missed hook shot by Laker Kareem Abdul-Jabbar, San Antonio's George Gervin made a basket that brought his team to within one.

For the second straight time down the floor, the Lakers could not score and a rebound gave the Spurs a chance to pull off a comeback victory. After a timeout with 10 seconds to play, guard Johnny Moore took the ball and drove into the lane. As the Lakers converged, Moore fed off to Mitchell, who went up for a short jumper but was surrounded by three Lakers and his shot was partially blocked by Johnson. Gilmore retrieved the loose ball, but before he could do anything the buzzer sounded.

"We decided we weren't going to give them anything, but we didn't really have much time to discuss it," Lakers Coach Pat Riley said.

Johnson scored only two points, but made 15 rebounds and 16 assists.

Abdul-Jabbar paced the Lakers with 28 points and Gervin had 25 to lead the Spurs, but down the stretch the key points for Los Angeles were provided by Nixon and Michael Cooper.

San Antonio trailed by as many as 13 points early in the second half, but closed the deficit to five by the start the final period. Los Angeles got back to an eight-point lead with 7:56 to play and then Abdul-Jabbar went to the bench for a brief rest. The Spurs quickly closed to within four.

At that point, Abdul-Jabbar came back in and, following a timeout, San Antonio had the ball and a chance to get to within two. Nixon stole the inbounds pass and drove for a layup that gave Los Angeles some cushion for the final minutes.

Cooper had made two free throws to give Los Angeles its eight-point bulge and then, after San Antonio had moved to within 98-94 with 2:22 remaining, Cooper drove the lane for a layup at the two-minute mark.

The Lakers played the second half without Bob McAdoo, who pulled a leg muscle in the final seconds of the first half. McAdoo tried to play early in the third quarter, but couldn't. His status was unknown for Sunday's game.