Dallas ? From classy David Robinson to unflappable Tim Duncan to charming Frenchman Tony Parker, the San Antonio Spurs are filled with model citizens.

That’s fine off the court.

On it, Malik Rose thinks his team needs to show more of a mean streak.

“It’s like, once we have a team down, we tend to give them life by not putting our foot on them and keeping them down,” Rose said. “We smell blood, we’re supposed to be going for the kill. But we have had tendencies to relax.”

The Spurs did it again Tuesday night against the Dallas Mavericks in Game 5 of the Western Conference finals.

In a game that could have clinched a spot in the NBA Finals, San Antonio was up by 19 points in the second quarter and still ahead 17 in the third, but wound up losing by 12, setting up Game 6 tonight in Dallas.

Although the Mavericks still trail in the series 3-2 and are likely to again be without leading scorer and rebounder Dirk Nowitzki, they’re still playing. And that’s all they care about.

“We feel as though anything is possible,” Dallas point guard Steve Nash said. “If we just stay with it and allow ourselves to be in the game, we’ve got enough guys with the toughness to come through.”

Dallas center Raef Lafrentz (45), guard Michael Finley (4) and guard Nick Van Exel celebrate in the final minutes of their 103-91 win over San Antonio. The Mavericks beat the Spurs Tuesday in Dallas.

Dallas knows a team facing elimination can win three straight because Portland did it against the Mavs in the first round. Dallas recovered to win Game 7, and won a deciding game against Sacramento in the second round.

The Mavericks likely will have to stretch the series another game for Nowitzki to return. The left knee he sprained in Game 3 was still achy Wednesday and coach Don Nelson said his 7-foot All-Star was “very doubtful” for Game 6. Nowitzki listed himself as a game-time decision.

The Spurs know all about losing a game they should have won. Of their six losses this postseason, they held a double-digit lead in five. Their biggest collapse, though, came in a victory, when a 25-point lead over the Lakers was cut to two and Robert Horry’s potential game-winning three-pointer rimmed out at the buzzer.

The only upside of the Spurs’ failure to close out big leads is that they’ve always bounced back to win the next game.

“We’ve been in and out with the intelligent quotient so far during these playoffs,” Robinson said. “We’ve got to be smarter. There’s no room for error now.”

What happened Tuesday night had to be seen to be believed.

The Spurs led 48-29 in the second quarter and 70-53 almost midway through the third.

Dallas took control with a 21-11 run at the end of the third quarter and a 23-5 spurt to start the fourth. With the Mavericks collapsing their defense around Duncan and forcing anyone else to beat them, nobody seemed interested.

The perimeter players seemed content to dribble out the clock and pass the ball around instead of putting up open shots. The sluggishness led to one basket on their first 12 shots in the fourth quarter.

San Antonio finished 3-for-17 in the quarter, scoring just 10 points — two less than Van Exel had in the final 12 minutes.

“They won 60 games — this isn’t like it’s some Joe Blow team off the street who we just lost to,” Spurs coach Gregg Popovich said.