Defending NBA champions the Toronto Raptors dramatically avoided falling 3-0 behind in the Eastern Conference semi-finals thanks to OG Anunoby's game-winning shot against the Boston Celtics.

Down by two points and two games, the Raptors were staring at a third consecutive defeat but Anunoby nailed a buzzer-beating three-pointer to give Toronto a 104-103 victory in Game 3 of the NBA playoff series on Thursday.

Anunoby was mobbed by his team-mates in wild scenes at Walt Disney World Resort, where he caught a cross-court pass from Kyle Lowry and hit the three as time expired.

The Celtics had taken a two-point lead with a half-second remaining after Kemba Walker assisted Daniel Theis' dunk.

But now the Raptors only trail 2-1 heading into Saturday's Game 4, which is good news for Toronto as no team in NBA history have rallied from a 3-0 series deficit.

Meanwhile, Kawhi Leonard inspired the Los Angeles Clippers to a 120-97 rout of the Denver Nuggets in the Western Conference semi-final opener.

Leonard – a two-time NBA champion – had a game-high 29 points for the second-seeded Clippers in Game 1 in Orlando, Florida.

Lowry fuels rallying Raptors

Lowry celebrated emphatically when Anunoby nailed his shot from beyond the arc. Lowry posted 31 points, eight assists and six rebounds for the Raptors. Fellow star Fred VanVleet contributed 25 points and six assists. Walker had 29 points for the beaten Celtics.

Per STATS, Anunoby became the first player to hit a buzzer-beating three-pointer in the playoffs with his team down by two points since Damian Lillard did so for the Portland Trail Blazers against the Houston Rockets in 2014.

Murray unable to back-up heroics

Coming off a gruelling seven-game series, which he averaged 31.6 points and scored at least 50 points in the Utah Jazz matchup, Jamal Murray was limited to just 12 points on Thursday. He was only five-for-15 shooting in the loss to the Clippers. Star team-mate Nikola Jokic finished with just 15 points.

It was a tough outing for Boston's Marcus Smart, who was four of 15 from the field for 11 points in 37 minutes.

Kawhi with the slam

There was no stopping Leonard en route to the rim against the Nuggets.

Rockets face Lakers

The Rockets and Los Angeles Lakers will open their Western Conference semi-final series on Friday. Eastern Conference top seeds the Milwaukee Bucks, meanwhile, will fight to snap back-to-back defeats in Game 3 against the Miami Heat.