The Pacers had lost four straight games and were fading in the Eastern Conference playoff picture. The win moved the Pacers within 1½ games of the eighth spot in the East and gave them a much-needed shot of confidence.
The Spurs have the third-best record in the Western Conference, but the reeling Pacers beat them without top scorer Jermaine O'Neal, who sat out with a sore left knee.
"It just shows that they're fighting, and some people are openly questioning that," Indiana coach Rick Carlisle said. "I think tonight showed a lot in terms of their belief that this thing isn't over. We preach to our guys continuously, 'Hey, hang in there, give yourself a chance.' Some people are seeing it differently, but not me."
The Pacers prepared for their final shot trailing 99-98 with 6.3 seconds left. Tinsley, a 6-foot-3 point guard, caught the inbounds pass near the 3-point line. He drove from right to left before lofting the shot over the 6-11 Duncan's outstretched arm, off the glass and into the basket.
Tinsley, who finished with 20 points and eight assists, knew he was driving toward Duncan.
"I was just trying to size him up and get an angle so he couldn't really jump," Tinsley said. "I happened to get under his body before he could jump, and it was too late."
Without O'Neal, Carlisle said there was no doubt he was going to let Tinsley create.
"Right now, the ball should be in his hands because he's our best player," he said.
With the score tied at 98, San Antonio's Manu Ginobili drove to the paint and drew a foul with 6.3 seconds left. He missed the first free throw and made the second, but his miss gave the Pacers a shot at the win. Ginobili shot 4-for-11 from the field and 4-for-7 from the free throw line.
"For me, today wasn't a good day shooting, either from the field or the free throw line," he said. "I'm disappointed in myself."
The Pacers led 77-76 at the end of the third quarter, but San Antonio took an 85-84 lead on a jumper by Tony Parker with 7:08 to go.
The Spurs then went on a 7-2 run, including two jumpers by Jacque Vaughn, to take a 92-86 lead with 5:37 to go. After Mike Dunleavy scored for Indiana, Duncan scored on a tip-in, then a dunk to give the Spurs a 96-88 lead with 3:42 to go.
The Pacers tied the score at 98 on a 3-pointer by Troy Murphy that rattled in with 1:26 remaining.
Parker scored 22 points, Duncan had 17 points and 11 boards and reserve Michael Finley added 17 points for the Spurs.
The Pacers made 20 of 22 free throws while the Spurs made 22 of 33, one reason San Antonio coach Gregg Popovich was in a foul mood after the game. The Spurs are trying to catch Phoenix for the No. 2 seed in the West, and Phoenix had beaten Dallas earlier in the day.
"From our point of view, I thought it was a pathetic performance for a team that's trying to prepare to go into the playoffs," he said. "In the fourth quarter, I thought we played embarrassing basketball. It was a very disappointing loss. A pathetic, pathetic performance."
Without O'Neal, the Pacers got the balanced effort Carlisle has been looking for. Dunleavy scored 21 points, Murphy had 20 and reserve Ike Diogu added 18 points and a season-high 13 rebounds.
Diogu filled in for O'Neal and center Jeff Foster, who left the game in the first quarter with back spasms and did not return.
"When one of your big guns is out, you've got to step it up," Diogu said. "In this line of business, you always have to be ready to go. There wasn't room for any drop-off, so I just went in there to have fun."
Duncan spent much of the game complaining to the officials. He received a technical foul in the third quarter, and was still talking after the game.
"They were allowed to be very physical and got away with a lot of stuff," he said. "They used it to their advantage."
- O'Neal has missed five of the last 13 games with the knee injury. Carlisle said the Pacers aren't ready to consider shutting him down for the season.
- San Antonio F Robert Horry sat out with an abdominal contusion. He has missed three of the Spurs' last four games.
- San Antonio shot 70 percent from the field in the first quarter.
- The Pacers are 3-8 when O'Neal doesn't play.