The Los Angeles Clippers defeated the Dallas Mavericks 118-110 in a thrilling first-round play-off clash.
After giving up a 10-lead early, Dallas, inspired by Luka Doncic, fought back to head into the second half up 69-66.
Despite Doncic finishing with 42 points, the most ever by a player in his first post-season game, Leonard's experience ultimately saw the Clippers over the line.
Mitchell stepped up to the plate in the absence of key player Mike Conley, proving nearly unstoppable with the ball in hand.
However Denver refused to go away, withstanding Mitchell's onslaught to force the game into overtime.
Jamal Murray then took over for the Nuggets, scoring 10 of his 36 points in extra time and giving Denver a 1-0 lead in the series.
The centre dominated as he racked up 26 points, but the Sixers were left to rue some sloppy handling which prevented them from sustaining any sort of meaningful run.
Jayson Tatum and Jaylen Brown shared the bulk of the scoring for Boston, combining for 61 points in the 109-101 victory.
The Toronto Raptors continued to show why they are the reigning champions with a dominant 134-110 win over the Brooklyn Nets.