Needing a win to keep the best-of-seven series alive, Oklahoma City appeared to be on their way to forcing a game six after overturning a nine-point deficit at the end of the third period to open up a 15-point lead in the fourth quarter, AFP reported.
But a sensational burst of scoring hauled the Blazers back within striking distance as their home fans at the Moda Center roared them on.
Lillard leveled the score at 115-115 with a driving layup and when Westbrook subsequently fluffed a chance to restore Oklahoma City's lead with 18.3 seconds left, Portland ace Lillard was left with a chance to grab the win.
The 28-year-old point guard took his time, eating up the final few seconds before sinking a three-pointer from long-range to seal a 118-115 win and a 4-1 series victory.
"Sometimes it's your turn to go through something, and when you keep fighting, and keep working through it, and stay together, there's a reward waiting for you," Lillard said after the win.
Earlier, Lillard had kept Portland in contention almost single-handedly, playing all 24 minutes of the first half and knocking down 12 of 18 shot attempts to score 34 points by the break.
His first-half exploits appeared to have caught up with him as the Blazers flagged in the third period.
But the Portland talisman led a mesmerizing late rally to finish with 50 points, seven rebounds and six assists.
Maurice Harkness added 17 points, Enes Kanter added 13 and C.J. McCollum finished with 17.
It was a disappointing end to the season for Westbrook, who added a triple double comprising 29 points, 11 rebounds and 14 assists.
Paul George led the Thunder scoring with 36 points, adding nine rebounds and three assists in a losing effort.
Lillard meanwhile revealed that Portland had been motivated by Oklahoma City's exuberant celebrations after their win in game three.
"They were out there doing all these celebrations," he said. "After one win, that was what they decided to do. We were like, OK, we want to win four games, and then there won't be nothing to talk about."
Portland will face either Denver or San Antonio in the next round. The Nuggets took a 3-2 lead in their series after a 108-90 win earlier on Tuesday.
Sixers, Raptors romp
In the Eastern Conference, the Toronto Raptors and the Philadelphia 76ers set up a heavyweight playoff battle after completing comfortable series victories.
The Sixers sealed a 4-1 series win after routing the Brooklyn Nets 122-100 while the Raptors swept to an emphatic 115-96 dismissal of the Orlando Magic to also win their series 4-1.
Joel Embiid led the Sixers with 23 points at Philadelphia's Wells Fargo Center.
The Sixers were always in control after surging into a 17-point lead at the end of the first quarter.
They led 60-31 at half time as Brooklyn's challenge effectively collapsed.
Embiid was again immense for Philadelphia, hauling in 13 rebounds to go with his points tally.
Five other Philadelphia players made double figures, with Ben Simmons scoring 13 points and Tobias Harris 12. JJ Redick and James Ennis III added 11 apiece. Jonathon Simmons finished with 10 from the bench.
Embiid believes the Sixers have the talent to mount a championship-winning playoff campaign.
"We think we can win it all," Embiid said. "Obviously, it's going to take a lot. You've got some great teams in the league, we're about to play one of them," the Cameroon international added.
"We just gotta take it one game at a time but we understand that we got all the talent we need, especially to win it all."