Halfback Jock Madden was the hero for the Tigers on Sunday, kicking a 79th-minute field goal to snap a eight-game losing run.
The Tigers' finals hopes ended months ago and they were ranked $10 outsiders at Allianz Stadium.
But led by co-captain Api Koroisau, Benji Marshall's side produced one of the upsets of the season to turn the make-up of the top four on its head.
With two rounds remaining, the Roosters (38 points, +162) sit third after blowing a golden chance to move clear of the ladder-leading Warriors (38 points, +295) and second-placed Penrith (38 points, +294).
"It was a poor game from us," said Roosters coach Trent Robinson, adding the result "wasn't good" for his side's top two hopes.
"There's two games to go and it's pretty clear.
"In the end we didn't win today and the ladder is where it's at and we know what we want to do.
"We still want to be the best team in the comp this year and the opportunity is still sitting right there."
In further bad news for Robinson, lock Victor Radley was placed on report and underwent a late head injury assessment.
The Roosters coach was unsure post-match if Radley had passed his concussion test.
Wests Tigers had every reason to not aim up on Sunday, especially when prop Alex Twal was ruled out with a head knock inside 60 seconds and Roosters winger Billy Smith touched down after just three minutes.
But unlike the past few months when they would have gone to water, the Tigers stood up for themselves.
"I reckon that's as proud as I've ever been as a coach, given we lost Twaly who is probably our most experienced player," Marshall said.
"We had guys, without sugar-coating it, who were on development deals playing against the team that was coming first.
"I'm just proud, proud of the effort, and I'd probably go as far as saying that as a club that's probably one of our best wins we've had."
Smith's opener sparked the Tigers into life with Patrick Herbert setting up winger Jeral Skelton for two tries and back-rower Tony Sukkar for one inside 26 minutes.
Madden's inability to nail any of the three conversions meant Sam Walker was able to tie the game up at 12-12 at halftime when he converted Smith's second.
Tries to Solomone Saukuru and Koroisau, who took charge of the goalkicking and booted a 40/20, pushed the Tigers further ahead.
Robert Toia crashed over for the home side but Walker rattled the conversion into the left upright, giving the Tigers 15 minutes to defend an eight-point lead.
The Roosters looked to have finally found their mojo after Mark Nawaqanitawase had a try chalked off before setting up Walker with a brilliant offload that cut the deficit to two.
After Hugo Savala was tackled off the ball, Walker made it 24-24 with a penalty goal with less than five minutes remaining.
A blunder from Roosters prop Naufahu Whyte put the Tigers back on the attack and when Radley was ordered off for his concussion test, they had time to get in position for a field-goal attempt.
Madden, who has missed the past month after a haematoma in his nether regions, calmly delivered.
"It was quite ballsy for him to play today," Marshall said with a wink.
"I'm really proud. Under pressure, to kick that to win the game was awesome."
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