BOSTON - Tim Duncan thought his latest victory over Boston should have been easier. Duncan remained unbeaten against the Celtics, finishing with 21 points and nine rebounds to help the San Antonio Spurs hold on for a 93-89 victory Monday night.

Manu Ginobili and Tony Parker each scored 15 points and Michael Finley had 12 for the Spurs, who have won 18 straight against the Celtics and nine in a row in Boston. The Celtics last beat the Spurs on Jan. 8, 1997, the season before Duncan entered the NBA.

Still, Duncan was frustrated with his team's performance.

"Wins are wins, but it's frustrating not to close people out," Duncan said. "It shouldn't have been that close. We allowed them to get back in the game."

The Spurs led by as many as 23 in the third quarter and 77-63 after the period. But after the teams traded baskets for much of the fourth, Boston used a late 13-0 run to make it close.

Delonte West's jumper from just inside the 3-point line pulled Boston to 91-89 with 8.7 seconds left, but Finley made two free throws with 3.2 seconds left to ice it.

"When you get it down to five points with two minutes to go, it becomes a game within a game," West said. "At that point, we made too many mistakes that cost us. That's what a good team does ¡ª they made you pay."

West led Boston with a season-high 27 points, while Al Jefferson added 26 points and 14 rebounds and Ryan Gomes scored 20 points. The Celtics have lost eight straight and are 2-14 without star guard Paul Pierce, who has been out since Dec. 22 with a stress reaction in his left foot.

It was the second straight night San Antonio blew a big lead. In Sunday's 99-85 victory in Philadelphia, the Spurs led by as many as 22, but allowed the 76ers to climb within five in the fourth before pulling away late.

"For stretches we're playing well," Duncan said. "We're defending well, we're moving the ball really well. But for stretches there, we're turning the ball over and not playing well defensively. It's just about getting a little more consistent and turning those stretches into quarters and those quarters into games."

Duncan said the next few weeks will gauge how good a team the Spurs really are. Before next month's All-Star weekend, San Antonio has road games against the Los Angeles Lakers, Utah Jazz, Phoenix Suns, Washington Wizards, Orlando Magic, Miami Heat, New Jersey Nets and Detroit Pistons.

"We should be playing better at this point," Ginobili said. "I'm looking forward to that trip. Usually that's when we make that step."

Duncan shot 7-of-17 from the field and 7-of-7 from the free throw line as the Spurs won their third straight and eighth in 10 games. His 10 second-quarter points helped San Antonio outscore Boston 30-14 after falling behind 24-20 through one.

The Spurs allowed early Boston runs of 7-0 and 6-0 and trailed 20-12 late in the first quarter. But after Spurs coach Gregg Popovich took out San Antonio's entire starting lineup, the Spurs went on a 15-4 run spanning the first and second quarters to go up for good. Brent Barry and Francisco Elson combined for 13 points during that span.

A pair of free throws by Elson gave the Spurs a 25-24 lead with 9:49 left in the second, and Finley added a fast-break layup the next time down the floor to cap the stretch.

Gerald Green's jumper pulled Boston back within 27-26, but the Spurs responded with another 11-2 spurt. Duncan scored six during the stretch, and Parker's two free throws put the Spurs up 38-28 with 5:43 left in the half.

San Antonio extended its lead to 50-38 by halftime.

"That's a team you can't make any mistakes against," Jefferson said. "Every mistake you make, they're going to make you pay for it. Smart championship team, MVP player ¡ª you can't make no mistakes against them."