Isiah Thomas scored 19 of his 30 points in the first half and had 20 assists to fuel the Detroit Pistons' 121-98 rout of the Los Angeles Lakers yesterday.

Thomas also excelled defensively as a near-sellout crowd of 23,475 turned out in the Pontiac Silverdome, and the Pistons lifted a local blackout for the nationally televised contest. They won their fifth game in the last six; the Lakers, leading the Pacific Division, lost only the second of their last 13.

The game was a homecoming for two Lakers, Michigan State's Magic Johnson and Michigan's Mike McGee, and they were Los Angeles' high scorers with 22 points apiece. James Worthy had 20 for the Lakers. Dan Roundfield played his best game at both ends for Detroit and had 20.

Detroit took control by outscoring the Lakers, 19-8, in the last 5 1/2 minutes of the first half to lead, 62-46.

The Lakers shot under 50 percent from the floor for the first time in 13 games.

Nets 100, Rockets 99: Otis Birdsong made a 20-foot fallaway jumper from the base line with one second left, giving New Jersey victory in the Meadowlands.

Houston, missing a chance to increase a 99-98 lead when Robert Reid failed on two free throws and Ralph Sampson shot an air ball, saw the Nets take possession and retain it with three seconds left despite a miss by Birdsong. He then scored off an inbounds pass.

Houston took three full timeouts and a 20-second timeout after Birdsong's go-ahead shot, but Mitchell Wiggins, who led a fourth-quarter Houston rally, missed a three-point attempt.

The Nets rallied from a 68-49 deficit to 90-79 with eight minutes left, then used a 12-0 run, the last six by Wiggins, to take their first lead all game, 91-90. Thomas, Pistons Run Down Lakers by 23 From News Services

Isiah Thomas scored 19 of his 30 points in the first half and had 20 assists to fuel the Detroit Pistons' 121-98 rout of the Los Angeles Lakers yesterday.

Thomas also excelled defensively as a near-sellout crowd of 23,475 turned out in the Pontiac Silverdome, and the Pistons lifted a local blackout for the nationally televised contest. They won their fifth game in the last six; the Lakers, leading the Pacific Division, lost only the second of their last 13.

The game was a homecoming for two Lakers, Michigan State's Magic Johnson and Michigan's Mike McGee, and they were Los Angeles' high scorers with 22 points apiece. James Worthy had 20 for the Lakers. Dan Roundfield played his best game at both ends for Detroit and had 20.

Detroit took control by outscoring the Lakers, 19-8, in the last 5 1/2 minutes of the first half to lead, 62-46.

The Lakers shot under 50 percent from the floor for the first time in 13 games.