BOSTON — After a record-setting seven overtimes in the first six games, the Celtics made an early night of it with a dominating stretch in the second.

That's second quarter, not second OT.

Ray Allen followed his 51-point Game 6 performance with 23 on Saturday night, Paul Pierce added 20 and Boston pulled away from Chicago just before the half to finish the Bulls off 109-99 — a rare regulation victory in what might have been be the best first-round playoff series in NBA history.

The Celtics will play Orlando in the Eastern Conference semifinals starting Monday. The seventh-seeded Bulls return to Chicago knowing they took the defending NBA champions to the limit — and quite often beyond.

Ben Gordon scored 33 for Chicago and Kirk Hinrich scored 14 of his 16 in the fourth quarter to help Chicago cut it to three points — thanks in part to a bizarre scoring change that added a point to the Bulls' score two quarters after a first-half 3-pointer was mistakenly ruled a 2.

But Boston made all 11 of its free throws in the last 2 minutes to hold on and keep its drive for an 18th NBA title alive.

Kendrick Perkins had 13 rebounds, Rajon Rondo had 11 assists and Eddie House scored 16 points — going 5-for-5 from the floor, including four 3-pointers.

After three consecutive overtime games, the series went from Odyssey to oddity when an unusual scoring correction helped the Bulls cut the deficit to three points in the fourth quarter.

With 5:44 left in regulation, the public address announcer said that a "technical error" in the first half had credited Gordon with a 2-pointer instead of a 3-pointer. Officials can use video replay to check whether a shot is from beyond the arc or not, but it is supposed to come at the first break after the basket — not two quarters later.

The change made the Celtics' lead 89-84, then Gordon hit a pair of free throws. Perkins followed with a layup and then traded free throws with Gordon before Eddie House hit a 3-pointer to make it 96-88 with 2:30 left.

The Celtics made 11 straight free throws from there — one of them on a three-point play by Allen that made it 105-97 with 37 seconds left.

Chicago led by as many as nine points, 32-23, after Gordon scored the first five points of the second quarter — giving him 17 at that point. The Bulls took a 36-30 lead with 8:01 left in the half before Boston scored the next 11 points and 20 of the next 22 to go into the break with a 14-point lead.