A resurgent James Harden led the way with 42 points and scored a last-gasp three-pointer as the Philadelphia 76ers defeated the Boston Celtics 116-115 in overtime on Sunday to square their NBA Eastern Conference playoff series.

Harden drained a 24-foot corner three with just 19 seconds remaining in overtime at the Wells Fargo Center for a win that leaves the best-of-seven series finely balanced at 2-2.

In a nerve-shredding finale, Boston's Marcus Smart nailed a potential buzzer-beating basket but it did not count as he was unable to get the shot off before time expired.

That left Philadelphia celebrating a gutsy victory that owed everything to the majestic performance of Harden, who bagged his second 40-point game of the series.

"I put the work in and it was all about being aggressive," said Harden afterwards. "Tonight it was really do or die."

Harden — who scored just 28 points across games two and three – rediscovered his shooting form just in time as the Sixers fought back from a late attack of the jitters to claim a precious victory.

The game had gone into overtime after a frenetic fourth quarter that saw Boston come roaring back from a nine-point deficit to take a 105-100 lead with just over two minutes remaining after Malcolm Brogdon knocked down a 25-foot three-pointer.

That hushed Philadelphia's fans but the Sixers clawed their way back to parity after a Harden layup and three unanswered points from P.J. Tucker.

Two Smart free throws put the Celtics back in front at 107-105 with less than a minute remaining, and Al Horford denied Embiid with a superb block as the tension mounted.

But with just 16.4 seconds on the clock Harden tied it at 107-107 with a deft driving jump shot to set up a frantic finish.

A desperate attempted buzzer-beater by Smart bounced off the rim to send the game to overtime.

Once again Boston looked ready to claim a decisive 3-1 series lead when Jayson Tatum sank a 27-foot three-pointer to make it 115-113 Boston with 38.1 seconds left in overtime.

The Tatum basket was tinged with controversy as he appeared to commit a clear offensive foul, pushing off on Tyrese Maxey, but the score stood and the Celtics were on the brink of victory.

Harden, however, had other ideas and he nervelessly knocked down the winning three.

The series now heads back to Boston for game five on Tuesday.