DALLAS -- Just because San Antonio coach Gregg Popovich gave the big-ticket Spurs the night off didn't mean he was conceding the regular-season finale against the Dallas Mavericks.

Those that were left were there to win.

A ragtag version of the Spurs rallied from 18 points down to beat the Mavericks 96-91 on Wednesday at American Airlines Center.

With six of the main Spurs left behind in San Antonio, little-used rookies such as center Boban Marjanovic and guard Jonathan Simmons led an unlikely comeback against a Dallas squad largely intact.

"These guys deserve a lot of credit for their competitiveness tonight," Popovich said. "They went through a bad stretch in the first half and worked themselves out of it starting in the second half by being steady, by being solid, and by not turning it over, communicating and sticking together. They did a fine job. I was real proud of them."

The loss meant the Mavericks (42-40) couldn't finish any higher than the sixth playoff seed in the Western Conference. Dallas came into the game with an outside shot of moving up to fifth, but needed to beat the Spurs.

The Mavericks, the No. 6 seed, will open the postseason at Oklahoma City.

Just getting to the postseason was a task for the Mavericks. Dallas came into Wednesday having won seven of its previous eight games to qualify.

"I just love the way this team has fought for the past two-plus weeks," Mavs coach Rick Carlisle said. "Tonight was disappointing, but we've got to flush it and get ready to for the playoffs. That's where we are."

First-round openers are Saturday and Sunday.

Game No. 82 had virtually no meaning for the Spurs, who had long since clinched the West's second seed and the best record in franchise history. San Antonio (67-15) will play Memphis in the first round.

"We're feeling good," Spurs veteran forward Boris Diaw said. "I think we're ready for the playoffs. We spend pretty much all season getting ready for it. We were just trying to get in the best position possible and get ready mentally and physically for the playoffs. I think we're in good shape."

Playing on the second half of a back-to-back, Popovich elected to rest the majority of his top guns. Six regulars stayed home, including MVP candidate Kawhi Leonard, LaMarcus Aldridge and Tony Parker.

What was left of the Spurs put up a valiant fight. Despite being down 52-34 at the half, San Antonio pulled even in the third quarter and took a 78-77 lead on Danny Green's 3-pointer midway through the fourth.

San Antonio held on down the stretch to sweep the four-game season series against its Southwest Division rival.

"We got bombed," Carlisle said. "We were struggling to make shots and our defense was poor. Bottom line, their second half was better than our first half. We played a great first half, and they played a better second half."

Marjanovic, filling in for Tim Duncan, scored a season-high 22 points on 7-of-9 shooting and 8 of 8 from the line, and grabbed 12 rebounds. Simmons also had the best outing of his young career with 19 points. Small forward Kyle Anderson scored 15 and Green had 12.

Point guard Raymond Felton scored 22 off the bench to lead Dallas, with Dirk Nowitzki adding 19 points. Nowitzki made only 5-of-17 shots as the Mavericks shot 40.3 percent.

NOTES: Dirk Nowitzki became the seventh player in NBA history with 7,000 made free throws. ... Spurs coach Gregg Popovich also gave G Manu Ginobili and F David West the night off. Popovich was asked why he didn't skip the regular-season finale. "You know, I thought about it," he said. "I didn't think it'd look good. It might look like don't give a (expletive). And I would never want to portray that kind of attitude." ... The Mavericks were without G J.J. Barea (groin) and G Devin Harris (thumb). Mavs coach Rick Carlisle is "optimistic" Barea will be fully recovered for the playoffs. ... Dallas owner Mark Cuban on Kobe Bryant: "I'm a huge Kobe fan now that he's retiring."