BOSTON – It started with a 3-pointer by Avery Bradley 16 seconds into the game. It ended with a layup by Jason Terry with 6.5 seconds left.

Yes, the Boston Celtics finally got their offense going.

“It’s coming,” Terry said. “You can feel it.”

It was there Sunday — for much of the game, anyway — when the Celtics beat the New York Knicks 97-90 in overtime to avoid being swept in the opening round of the playoffs.

But if it’s not there Wednesday night in New York, the Celtics will have a very tough time forcing a sixth game Friday night in Boston.

The Celtics scored fewer than 20 points in six of the first 10 quarters in the series. They scored 78, 71 and 76 points in the first three games after scoring fewer than 80 just five times during the regular season.

“We’ve got a lot of basketball in us,” Terry said, “but there’s always something like one quarter that holds us back. So if we can put together four quarters of great Celtics basketball, ball movement, getting out in transition, then this series is going to be a long one.”

The Celtics made 51.3 percent of their shots in the first half Sunday after averaging 39.5 during the first three games. But they dropped to 25 percent (4 of 16) in the third quarter when the Knicks cut a 59-39 deficit to 68-65 entering the fourth.

And they finished with just three offensive rebounds — and only two second-chance points — in 53 minutes.

But Boston managed to win when New York’s own shooting woes continued in overtime with only two field goals in eight attempts. Carmelo Anthony hit just 1 of 4 shots in overtime and finished at 10 of 25 despite leading all scorers with 36 points.

“We didn’t shoot the ball well,” Anthony said, “and we still put ourselves in a position to win the basketball game. There’s an upside to that.”

The downside for him was the Celtics’ defense, particularly that of Brandon Bass. Before fouling out with 4:27 left in the fourth quarter, he kept the NBA’s leading playoff scorer from getting hot. Anthony missed all seven of his 3-pointers.

“(Bass) was the star of the game,” said Boston Coach Doc Rivers.

The Knicks got back in the game after Anthony picked up his fourth foul with New York trailing 65-51 with 3:35 left in the third quarter. With Anthony on the bench, Raymond Felton scored 11 points in a 14-3 run that cut the deficit to 68-65.

“The fact that we came back from a 20-point deficit in that building shows a lot about our team,” Felton said.

When they get to their own building, they’ll have their second-leading scorer back. J.R. Smith, the NBA’s Sixth Man of the Year, was suspended for Sunday’s game for elbowing Terry with an elbow in New York’s 90-76 win in Game 3. Smith is averaging 16.3 points per game in the series.

Smith’s perimeter shooting will put more pressure on the Celtics’ defense and, perhaps, open up the lane for the Knicks’ offense.