Before the Eastern Conference semifinal series with the 76ers, much was made of the physical toughness of the Boston Celtics, and rightly so.

Yet, the Celtics have a mental resolve that was tested to the limit during Saturday's 101-98 overtime win at the Wells Fargo Center that gave them a 3-0 stranglehold over the Sixers in their best-of-seven series.

The Celtics took an 89-87 lead with just 1.7 seconds left in regulation after Terry Rozier stole the ball and fed Jaylen Brown for a layup.

Boston, which had lost six straight road games – the last three in the regular season and three in their first-round playoff series with the Milwaukee Bucks – was less than two seconds from taking a commanding lead.

Then, on the inbounds play inside half court, Ben Simmons found an open Marco Belinelli, who hit a turnaround two-pointer in the corner to send the game into overtime.

Most teams would be deflated at that point.

The Celtics response?

"We were not deflated, even a little bit," said Marcus Morris, the former Philadelphia high school star at Prep Charter, who had nine points, seven rebounds, and three assists. "We were happy."

Happy?

"Then we knew we could beat them in overtime."

Even Terry Rozier wasn't down, and he had the worst view of Belinelli's last-second shot. Rozier was guarding Belinelli, and he tripped and watched from the ground as the shot went in.

"It was a moving screen, and I got hit and fell, and I have to do a better job with him," said Rozier, who had 18 points.

An arena worker thought that Belinelli hit a three and fired off a confetti gun, which caused a delay.

Rozier realized the confetti was premature.

"I knew it wasn't a three. That was my man who hit the shot, and I had to watch him hit the shot on the ground," Rozier said. "I wasn't worried about it being a three."

Even that delay didn't bother the Celtics.

"It took about 10 minutes to clean up the confetti, and I thought it was funny," Morris said. "It provided a little more motivation."

Not that the Celtics needed much more to be motivated about. The chance to break open this series was motivation enough.

So the Celtics kept their poise, but not the lead.

It wasn't long before the Celtics found themselves down 94-89, when JJ Redick hit a three-pointer with 3 minutes and 41 seconds left in overtime. The Sixers still led, 96-92, after Redick hit a 15-footer with 1:44 left, but the Celtics kept charging.

"I have never been around a group of guys, and I have been around some really special ones, that can just turn the page," Boston coach Brad Stevens said. "They just turn the page and play the game the right way."

The Celtics didn't even grab their first overtime lead until Al Horford's layup off an outstanding inbounds pass from Morris made it 99-98 with 5.5 seconds left. Horford then stole the ball, was fouled, made both free throws, and the Celtics survived a missed Belinelli three-point attempt to earn the win.

"They made a tough shot to go into overtime, but unfortunately we have been there before," said rookie Jayson Tatum, who had 24 points and was plus 24 for the game. "We know how to respond, and that is what we did today."

The frustration on the road in the first round benefited the Celtics on Saturday in Philadelphia.

"I think we learned a lot from the first round, playing on the road," Horford said. "We just stayed with it and kept fighting, and I felt like we made a lot of winning plays down the stretch."