Replacement winger Dom Shipperley scored a 78th minute try to snatch the unlikely victory for the Reds over the Force.
Victory moved the Reds to 62 points and all but guaranteed they will qualify in the top-two and have home advantage throughout the playoffs.
Wallabies coach Robbie Deans, who was watching the game, however faces a major injury concern heading into the international season over veteran lock Nathan Sharpe, who twisted his right ankle.
The Force captain, who has 93 caps for the Wallabies and was a certainty for their World Cup squad, told Australian television he thought he had sustained ligament damage but would need a more comprehensive examination on Sunday.
The Force dominated the first half with centre Rory Sidey scoring two tries after sustained pressure from the home side while both of the Reds' tries came against the run of play.
Flyhalf Quade Cooper, whose defensive frailities have forced the Reds to hide him on the wing when the opposition have the ball, opened his side's account when he rushed up and intercepted a pass while the Force were hot on attack and sprinted 80 metres to score.
Scott Higginbotham's try, which gave the Reds the lead for the first time, was just as fortuitous when he scooped up the ball after it had been kicked through the scrum, ducked under Force scrumhalf Brett Sheehan's tackle and ran 25 metres.
Force fullback James O'Connor, who had kicked an early penalty to open the scoring before he missed both conversion attempts, slotted his second penalty in the 35th minute to give his side a 16-14 lead at the break.
Cooper kicked an early second half penalty to give the Reds a 17-16 lead and the remainder of the match was virtually played between the 22-metre areas before Force flanker David Pocock scored in the corner in the 68th minute.
The Reds then smashed away at the Force line for up to 20 phases, before Cooper threw a wide pass to Shipperley who crossed in the corner. Cooper converted from the sideline to give the Reds a tight win.
"We knew it would be tough, these debries always are," Reds captain James Horwill said. "We lacked a bit of finish there and gave away a lot of penalties.
"But I'm really happy we got the result in the end."
Scorers:
Force - Tries: Rory Sidey (2), David Pocock; Penalties: James O'Connor (2)
Reds - Tries: Quade Cooper, Scott Higginbotham, Dom Shipperley; Conversions: Cooper (3); Penalty: Cooper
Teams:
Force – 15 James O’Connor,14 Nick Cummins, 13
Patrick Dellit, 12 Rory Sidey, 11 David Smith, 10 James Stannard, 9
Brett Sheehan, 8 Richard Brown, 7 David Pocock, 6 Matt Hodgson, 5 Nathan
Sharpe , 4 Sam Wykes , 3 Matt Dunning , 2 Nathan Charles , 1 Kieran
Longbottom
Subs: 16 Ben Whittaker, 17 Tim Fairbrother, 18 Tom Hockings, 19 Ben McCalman, 20 Mark Swanepoel, 21 Mitch Inman 22 Alfie Mafi.
Reds – 15 Jono Lance, 14 Luke Morahan, 13 Will
Chambers, 12 Ben Tapuai, 11 Rod Davies, 10 Quade Cooper, 9 Will Genia, 8
Radike Samo, 7 Liam Gill, 6 Scott Higginbotham, 5 James Horwill (c), 4
Rob Simmons, 3 James Slipper, 2 Saia Faingaa, 1 Ben Daley.
Subs: 16 James Hanson, 17 Greg Holmes, 18 Adam Wallace-Harrison, 19 Jake
Schatz, 20 Ben Coridas, 21 Ian Prior, 22 Dom Shipperley.