Dallas ? There are moments that can define not only a game, but also an entire playoff series.

Dirk Nowitzki and the Dallas Mavericks can hope that one of them didn’t happen in the fourth quarter Friday night.

As Tim Duncan was leading San Antonio on a 12-0 run to open the final quarter, Nowitzki went down with a sprained left knee. The Spurs controlled the game the rest of the way to defeat Dallas, 96-83, and take a 2-1 lead in the Western Conference finals.

“The news is not good,” Mavericks coach Don Nelson said. “His knee is very sore. I don’t expect he’ll play (in Game 4 Sunday), and I’m not sure if he’ll play in this series. We should not take a gamble or a chance on Dirk Nowitzki. I told Dirk he must feel he’s totally 100 percent for me to play him again this series.”

Duncan scored 14 of his 34 points in a lopsided fourth quarter that sent the fans streaming to the exits with more than three minutes left. Duncan had 24 rebounds, and Tony Parker scored 29 points for the Spurs.

It was a stunningly quick turn of events in a game that began with lead referee Dan Crawford joking to the team captains that he would let them call their own fouls, playground style.

Dallas had a few good moments — mostly in the second quarter — before having an extremely bad one midway through the fourth when Nowitzki banged his left knee against Manu Ginobili of the Spurs while going for a rebound.

San Antonio center David Robinson (50) tries to block a shot by Dallas guard Michael Finley in the second quarter of the Spurs' 96-83 victory. The outcome Friday in Dallas gave San Antonio a 2-1 series lead.

Nowitzki’s knee buckled, and he stayed down for about two minutes before being helped off the floor, unable to put any weight on his leg.

“It happened really quick,” Nowitzki said. “My knee was hurting pretty bad. I’ve never had a knee injury before, so really the pain is new to me. I couldn’t move my leg anymore.”

The last time there was such a scene at the American Airlines Center, it was Chris Webber of Sacramento being carried off the court with a knee injury during Game 2 of the second round. He never returned to that series, and the Mavs can only hope the same does not happen to Nowitzki.

The initial diagnosis was a sprain, and Nowitzki, who left the arena on crutches, was to undergo an MRI today. Game 4 is Sunday night.

“I hope he’s not out, because he’s a great player, and you want everyone having all their horses,” Spurs coach Gregg Popovich said. “But we can’t change, we can’t play differently.”

Duncan surpassed 30 points for the fourth consecutive game, and San Antonio outrebounded Dallas 59-36.