BOSTON – It’s not how you start a game, it’s how you finish.

That was the message Boston coach Doc Rivers told his team last night before the Celtics stormed the Garden to take on the Golden State Warriors.

Rivers said that the Celtics played poorly in the second half, and especially in the fourth quarter in their two previous games, resulting in losses to Atlanta and Indiana.

He wanted to make sure that didn’t happen again last night.

And it didn’t.

The Celtics outscored the road-weary Golden State Warriors who were playing their fifth consecutive game away from home, 31-19, in the third quarter to break open a close game, and 29-28 in the fourth to snap a two-game losing streak with a 109-95 triumph over the upset-minded Warriors.

“I’m concerned about finishes more than slow starts,” Rivers said.

In a game that featured four ties and eight lead changes, the Celtics only led 49-48 at halftime against a team with only eight healthy bodies that they expected to blow out.

“I think in the first half we were looking around at each other instead of just going out there and playing ball the way we know how to play,” Boston captain Paul Pierce said. “In the third quarter we picked it up and played our defense.”

Rivers said that it could be a number of things as to why the Celtics have struggled in the fourth quarter as of late, including fatigue, focus, and execution, but he is not sure what the main culprit is.

“Usually its focus and concentration,” Rivers said. “We get a lead and we lose it because we get comfortable. But we have to get to that point where we finish out things. We have to get the mental ability to be able to finish things when they’re going well for you.”

Rivers attributed some of the reason for the Celtics’ struggles as of late to implementing their highly-recruited new players into their system, and mixing the veteran talent with the youth movement.

“We did have a lot of games at the start [of the season] and we didn’t have a lot of time to work on things and we are starting to catch up on that,” Rivers said.

Boston’s second-half struggles were non-existent last night.

The Celtics used their two practices on Monday and Tuesday to shore up their defense and it showed last night. While the Celtics have limited teams to an NBA-low 88.27 points per game, they allowed Atlanta and Indiana to score at least 97 points. They limited the NBA’s highest scoring team in Golden State to 16 points below their average.

The Celtics turned up their intensity on defense in the third quarter and carried that momentum into the fourth.

After Corey Maggette (23 points, 8 rebounds) tied the game up at 55-55 for Golden State with a free throw, it seemed to awake a sleeping giant in the Celtics.

The Celtics shut down Golden State’s high-powered offense after that and went on an 11-0 run which gave them their biggest lead of the game up to that point, 66-55.

They would extend that lead to 80-67 by the start of the fourth quarter thanks in large part to Rondo, who scored 12 points in the third.

“[Rajon] Rondo really set the tone by getting in his game, pushing the ball and really being aggressive on offense,” Pierce said. “Even though the fourth quarter could have been better, the third quarter is something we can look at and sort of build on. I thought in the third quarter, the way we played was Celtic basketball and we just want to continue to play that way.”

“Rondo changed the momentum of the game,” Kevin Garnett said. “…He was aggressive. He picked up the energy…I just think he set the tone for the second half.”

Golden State made one last push, slashing Boston’s lead to 86-76 with 7:30 remaining, but Eddie House (11 points) answered with back-to-back 3-pointers for the Celtics to put the game out of reach.

All five of Boston’s starters scored in double figures, with Rajon Rondo (18 points, 12 assists, 7 rebounds), Paul Pierce (19 points, 4 rebounds), Ray Allen (15 points), Kendrick Perkins (15 points, 7 rebounds), and Kevin Garnett (12 points, 6 rebounds) leading the way.

Monta Ellis scored 18 points and Stephen Curry had 13 points for the Warriors.

The Celtics are still trying to put together a full 48-minute game, Rivers said.

“I don’t think we are a 48-minute team yet. We have to work to get there,” said Rivers.

“We know we are a better team as far as being solid and consistent,” Garnett said.

roblee@projo.com