The Celts’ Paul Pierce, center, snares a loose ball before the Hawks’ Marvin Williams can get to it during the first half last night. Trailing the play at left is Boston’s Leon Powe.

The Providence Journal / Glenn Osmundson

ATLANTA — A few hours before last night’s Game 3 playoff game against the Celtics, Atlanta Hawks’ rookie Al Horford watched footage of Muhammad Ali’s famous upset of George Foreman in Zaire.

If he needed any more motivation, Horford and the rest of his team were featured during a pregame video at Philips Arena that asked the hometown fans to “imagine if there were never any upsets?” and proclaimed “the best team doesn’t always win.”

The Hawks then proceeded to pull off a convincing, if not shocking, victory by beating the Celtics, 102-93. The win puts a new spin on the series with Boston now leading 2-1 heading to tomorrow night’s Game Four.

Coming in, the Hawks felt they’d played decent defense in the first two games of the series, but finding a spark on offense was clearly the Hawks’ top priority. Leading scorers Joe Johnson, Josh Smith and Mike Bibby combined to shoot 29 percent in the first two games but they reverted to form almost from the opening tip last night. Smith was the game’s best player as he flew to the rim and also made three 3-pointers on his way to a team-high 27 points. Johnson added five threes and 23 points and Bibby contributed 12 points and 8 assists.

“I’m sure the Celtics thought the series was over, but we got great crowd support and Joe got us going early and that momentum carried over all game,” said Smith.

If the Celtics had any dreams of cruising to a four-game sweep, they were rudely awakened last night.

“We know that it’s not going to be easy. That’s the thing we have to understand,” said Paul Pierce. “It’s going to be a battle. They’re not going to lay down for us. We have to go out there and take it. We have to play. Now we’ve figured that out.”

Kevin Garnett led the Celts with 32 points and 10 rebounds. Pierce added 17 points and Ray Allen chipped in 13, but the Celtics clearly lacked the defensive execution and productive bench play (a combined 11 points last night) it received in the first two games.

“As a team, we lost our composure at times. That’s out of character for us,” said Rajon Rondo. “We did some things we haven’t done all series.”

Most of those “things” focused on breakdowns at the defensive end of the floor. The Celtics were out-rebounded (43-35) and watched the Hawks make 47 percent of their shots.

The Hawks rolled to 56 first-half points and blew the game open with a dominating third quarter. That 12 minutes were easily the worst of the series for the Celtics, as not only did the defensive mishaps pile up but the offense slipped badly as well. The Celts shot 24 percent (5-of-21) and were out-scored, 28-18, in the quarter.

“They made shots and just played with a lot of energy,” said Boston’s James Posey. “We have to match that all game long. Tonight, we didn’t do that.”

Most of the second half was played without a shot clock. The disturbing development clearly bothered both teams, with the Celtics scoring just 37 second-half points.

The game was tied at 68-all with 6:31 to play in the third when the Hawks caught fire. Al Horford (17 points, 14 rebounds) and Johnson proved to be the chief tormentors as the rookie big man hurt both Kendrick Perkins and Garnett while Johnson continued making key jumpers. Two threes by Johnson helped the Hawks open a 82-70 lead late in the quarter and the Hawks were thrilled to carry a 84-74 lead into the final quarter.

A Smith 3-pointer pushed the lead to 89-74 early in the fourth and the Hawks protected a double-digit lead up until the final minute.

kmcnamar@projo.com