Sessions landed the go-ahead score that put the Lakers on top 89-86 with 48 seconds left, then reserve Blake made a dagger three-pointer on a pass from Kobe Bryant with 18 seconds remaining to bury the Nuggets.
"I was letting (Kobe) know (I was open) throughout the game, even when I missed one, I said 'I'm going to make the next one,'" Blake said after tallying eight of his 10 points in the fourth quarter.
Bryant scored 22 points to go with eight rebounds and six assists to leave the Lakers able to secure a second round spot with a win in Game Five on Tuesday in Los Angeles.
Oklahoma City Thunder await the winner of the series.
Denver were coming off a 15-point home triumph over the Lakers in Game Three but they failed to recapture their fast-paced style and allowed the Lakers to slow them.
Danilo Gallinari had a team-high 20 for the Nuggets, who led for much of a tight game, but were shot down in the end.
In the Eastern Conference, Boston and Philadelphia each seized 3-1 series leads while New York avoided being swept by Miami and cut their series deficit to 3-1.
In Boston, Paul Pierce shook off a first-half knee injury to score 24 points while Rajon Rondo sliced up the defence with 20 points and 16 assists as the Celtics blew away Atlanta 101-79.
Pierce briefly went down with a left knee injury in the second quarter, but returned after halftime to spark one of Boston's best offensive performances of the playoffs.
"Ball movement. We started with stops and played together," Rondo said after the Celtics shot 51 per cent and made 11 three-pointers.
In Philadelphia, the eighth-seeded 76ers moved within a game of eliminating the injury-depleted Chicago Bulls as they scored their third consecutive victory over the East's top seeds, winning 89-82.
A victory on Tuesday in Chicago would give the 76ers their first playoff series win in nine years. "I think we're confident," 76ers forward Elton Brand said. "But we don't want to come back for Game Six. We want to finish this."
Spencer Hawes paced the 76ers with 22 points, making nine of 11 shots from the field, and Jrue Holiday added 20 after a sluggish start.
Chicago hung in for much of the game despite playing without injured center Joakim Noah, who has a sprained ankle, and 2011 Most Valuable Player Derrick Rose, who is out for the season.