PHILADELPHIA -- Al Horford has one word to describe Boston Celtics coach Brad Stevens: genius.

Horford gushed about his coach's whiteboard prowess after Stevens twice drew up after-timeout plays that delivered layups at critical junctures of Boston's 101-98 overtime win over the Philadelphia 76ers in Game 3 of the Eastern Conference semifinal series on Saturday at Wells Fargo Center.

Stevens twice called mid-possession timeouts in order to draw up sets when his team looked disheveled. The first resulted in a Jaylen Brown layup to tie the game with 24 seconds to play in regulation; the second put Horford in position to muscle home a winning layup with 5.5 seconds remaining in overtime.

"Brad is a genius, man. Unbelievable," Horford said during an on-court interview with ESPN immediately after Boston's win to take a 3-0 series lead.

The Celtics trailed by a point when both Joel Embiid and Ben Simmons misfired with less than 21 seconds remaining in overtime. Stevens called consecutive timeouts with 8.4 seconds to go. Horford, posting up Robert Covington alone under the basket, caught a long lob from Marcus Morris and finished on the opposite side of the rim to put Boston out front 99-98.

"Sometimes he draws stuff up, and I look at it, I'm like [confused]," Horford said while playfully scrunching his face. "But Marcus delivered it, great pass, I missed some shots that I normally make there, but I made that one, so it was good."

Horford missed the front end of two free throws with a chance to tie the game a short time earlier. He then came up with a steal on the possession after his go-ahead layup and sealed the win with two freebies.