The Celtics’ Paul Pierce tries to remain in control of the basketball and stay inbounds as the Rockets’ Yao Ming defends during the second quarter last night at TD Banknorth Garden.

AP / Elise Amendola

BOSTON —The Celtics have fallen on hard times, and the rest of the league is taking full advantage of it.

They have quickly gone from being talked about as the greatest team in NBA history after beginning the season with an NBA-record 27-2 start, which includes a franchise-record 19-straight wins, to losers of six of their last eight games. The lastest came last night at the hands of the Houston Rockets, 89-85, at TD Banknorth Garden, only its second home loss of the season.

The Celtics (29-8) no longer have the best record in the NBA, and they have fallen behind Cleveland (28-6), whom they face tomorrow at the Quicken Loans Arena, for the best record in the Eastern Conference.

They are fighting with Orlando (28-8) for the second-best record in the East in the race for home-court advantage in the postseason.

There suddenly is a crack in the foundation. The Celtics, who pride themselves on defense, have not been able to make stops when it counts, and their offense has been stagnant in the fourth quarter of late.

That was the case last night against a Houston team playing without two of its starters — All-Star guard Tracy McGrady and forward Shane Battier.

Boston was outscored, 18-11, in the fourth quarter.

“We got great shots, they just didn’t fall,” Boston coach Doc Rivers said of the final period. “Eleven points in the fourth quarter is not going to win a lot of games.”

Houston snapped a five-game road losing streak with the victory.

“Our guys came out and competed for 48 minutes after we’ve had a rough road trip, and to come in here and beat these guys here, control the fourth quarter, just a terrific win,” Houston coach Rick Adelman said.

The Celtics entered the final session with a 74-71 lead, but the Rockets picked up their intensity on defense and outplayed the defending world champions down the stretch.

A Yao Ming fade-away jumper with 3:17 remaining snapped an 81-81 tie and gave the Rockets a two-point cushion.

On the Celtics’ ensuing possession, Houston star forward Ron Artest (17 points) fouled Paul Pierce, fouling out of the game with 3:04 remaining.

Pierce then buried a foul-line jumper with 2:43 left to tie the score at 83-83.

Houston answered when it passed the ball into the paint to Yao, and Yao was fouled by Kendrick Perkins on a hook shot. Yao only connected on one of the two free throws to make it 84-83.

Garnett then made a great bounce pass to Rajon Rondo, who was all alone underneath the basket. Rondo banked home a layup with 1:34 left to give Boston an 85-84 advantage.

Houston called a timeout to regroup, and they did just that.

After Pierce was called for a controversial offensive foul with 59.6 seconds left, Von Wafer (12 points) buried a 3-pointer from the right wing with 43.8 seconds left as Houston retook the lead, 87-85.

The Celtics called a timeout and called Pierce’s number. But his jumper over Wafer missed its mark, and Houston got the rebound.

“We got the shots we wanted,” Rivers said. “Paul Pierce in the middle of the paint, I think I’m taking all of those shots. He missed a couple.”

Yao then iced the game with a rebound and a put-back, scoring the final two of his team-high 26 points (along with 8 rebounds) with 12.9 seconds left.

“Yao is unstoppable,” Artest said. “Sometimes a center will have a good game against him, but realistically, Yao cannot be stopped.”

Boston’s Big Three provided the bulk of the offense. Pierce (26 points, 5 rebounds), Garnett (18 points, 8 rebounds), and Ray Allen (13 points, 6 assists, 5 rebounds) all played well on the offensive end, but Houston didn’t have any problems scoring after the first quarter.

The Rockets shot 48.6 percent from the field (35-for-72) for the game, and that includes a first quarter in which they connected on only 42.1 percent of their shots and were outscored by the Celtics 31-22.

Houston turned the tables on the Celtics in the second quarter, which the Rockets won, 30-18, to take a 52-49 lead into the locker room at halftime.

Boston outscored Houston, 25-19, in the third, before the Celtics fell apart in the fourth quarter.

“I thought our spirit was right,” Rivers said. “I thought our effort was right. We executed, we just didn’t make shots, make plays and get stops at the end.”

roblee@projo.com