A low-trajectory Jamie Soward field goal with less than 10 seconds left on the clock earned the Panthers an extraordinary 19-18 victory at Allianz Stadium after the minor premiers looked to have snatched the match just moments earlier.
A 74th-minute four-pointer to star halfback Mitchell Pearce, and a controversial conversion to five-eighth James Maloney - who was off the field when the try was scored - had given the Roosters an 18-12 lead.
But an incredible reply from the Panthers with three minutes remaining, followed by an ice-cool sideline conversion from Soward - and then his late, late heroics sent Penrith's travelling fans into a frenzy.
After an injury-ravaged campaign, the amazing victory left the Panthers one win away from a fairytale grand final appearance, while the Roosters return to Allianz on Friday night for a sudden-death showdown with Johnathan Thurston's resurgent North Queensland.
If they survive that, the Roosters will take on new title favourites South Sydney in a blockbuster grand final qualifier in two weeks.
Cleary said he could barely believe his side had ended the minor premiers' six-match winning streak after falling behind 12-4 early in the second half and then needing a spectacular try to Dean Whare in the shadows of fulltime - and then Soward's sideline leveller - to even stay in the match.
"I won't get much more fun than that," Cleary said.
"We just wanted to stay in it as long as we could. I guess we just kept doing that."
A premiership winner four years ago with St George Illawarra, Soward rated his match-winning 35m drop goal as probably the biggest play of his career.
Like Cleary, the game-breaking halfback said he could scarcely believe the Panthers were just one win away from their first grand final shot in 11 years.
But now the Panthers have come this far, Soward says they can go all the way and land a first title since an equally unexpected run to grand final glory in 2003.
"I guess with winning that game now, you've got to prepare. You're 80 minutes away from something you'll never forget," he said.
"The younger guys thought tonight was good. If we win next time around, it's the best week of your life."
One of those younger guys, 19-year-old-winger Dallin Watene-Zelezniak, pulled off a remarkable play at the death on Saturday night to set up Whare's second try in the 77th-minute to ensure the grandstand finish.
From a Soward grubber kick attacking the Roosters' line, Watene-Zelezniak somehow tapped the ball back from outside the field of play and Whare pounced to score.
Soward then nailed the conversion from out wide before booting the ultimate drop goal winner to seal a crazy victory.
From hot favourites to become the first team to secure back-to-back premierships in a united competition since Brisbane in 1993, suddenly the Roosters are now in a desperate fight to keep their title defence alive.
Compounding their woes was the first-half reporting of hard-man prop Jared Waerea-Hargreaves for a swinging arm that left Penrith prop Brent Kite dazed and sporting a black eye.
With Aidan Guerra only returning from suspension next week and Dylan Napa still having a game to serve of his two-match ban, the Roosters can ill afford to lose any more strike forwards at the judiciary.
