The Indiana Pacers pulled off an extraordinary fourth-quarter comeback to beat the New York Knicks 138-135 in overtime in Game One of the NBA Eastern Conference finals.

The Knicks, playing in the conference finals for the first time in 25 years, had been in charge of the contest at Madison Square Garden and led by 14 points with 2:51 left in the game before the Pacers suddenly came alive.

In stunning fashion, Indiana sank six straight three-pointers down the stretch, five of them from Aaron Nesmith.

The Pacers were an inch away from winning the game in regulation when, with the final play of the fourth quarter, Tyrese Haliburton backed out of the three-point arc and his shot while back-pedalling hit the rim and bounced in.

The Pacers, who were 17 points down at one stage in the fourth, celebrated what they thought was a famous victory.

But the video replay showed that Haliburton's foot was on the three-point line and the shot was downgraded to a two-point shot, meaning the game went to overtime.

The stunned Knicks regrouped and led by a point with 35.2 seconds of overtime remaining when Jalen Brunson made a driving, floating jump shot.

But a lay-up from Andrew Nembhard and then a six-foot dunk from Obi Toppin secured a thrilling win for the Pacers.

Haliburton top-scored for the Pacers with 31 points and 11 assists while Nesmith ended with 30 points on 8-from-9 three-point shooting.

"I'm so proud of the resilience of this group, we've shown it all year. We've had to win in so many different, random, unique ways and today we just kept going, kept fighting, and man, that's fun," Haliburton told broadcasters TNT.

The win was the Pacer's fourth comeback from 15 points or more down during the post-season.

"We played a lot of games where it felt like the other team had control," Haliburton said.

"It ain't over until it's over, until it hits zero. But I really do think there's a lot for us to improve on," he added, highlighting rebounding as an area they need to be better in.

Brunson led New York with 43 points while Karl-Anthony Towns added 35 points.

Knicks head coach Tom Thibodeau was keen to move on from the loss.

"There's obviously a disappointment when you fall short. We have to take a look at the film, make our corrections, and then we gotta change quickly for game two," he said.

"We gotta be ready for game two, and that's all you look at. So, the disappointment, we've got to turn that into more determination," he added.

Game two of the series takes place in New York on Friday. (AFP)