The Reds moved up to fifth place in the Super Rugby standings when they held on to beat the Waratahs 12-14 in Sydney.
Berrick Barnes missed a penalty kick two minutes from full time that could have won the match for the Waratahs.
Flyhalf Quade Cooper kicked two penalties, the only points of the second half, to secure the Reds' 10th win of the regular season, leading their rally after the Waratahs had taken a 12-8 lead to halftime.
The Reds' fifth placing ensured they will play seven-time champion Crusaders at home in Christchurch in next weekend's play offs after the Crusaders finished the regular season in fourth place.
The sixth-place Cheetahs, from Bloemfontein, will face the third-place team in the quarterfinals: an opponent still to be decided by matches involving the Brumbies and Pretoria-based Bulls.
If the Brumbies finish in third place they will play the Cheetahs next Sunday.
Due to the Super Rugby draw the Crusaders missed out on playing the Reds in 2013.
The Waratahs led 12-8 at halftime, but failed to score a point in the second half as Brendan McKibbin and Barnes missed four shots at goal between them to throw away an Australian derby victory.
Cooper, controversially passed over by former Wallabies coach Robbie Deans in this month's three-Test series against the British and Irish Lions, had a mixed night with a wonderful lofted pass to put Lucas over in the corner in the first half but also made several kicking errors.
Cam Crawford scored his fifth try in five consecutive outings for the Waratahs in the 12th minute before Lucas put the Reds in front 8-7 midway through the opening half.
The Waratahs were reduced to 12 men when hooker John Ulugia was sent to the sin bin after a tackle on winger Rod Davies.
McKibbin latched on to a Cooper pass and broke clear to send Barnes over in the corner to give the Waratahs the halftime lead.
Cooper kicked the Reds to the front with two penalties early in the second half before a concerted finish by the Waratahs threatened to take the game away from them.
But they defended resolutely with Wallaby open-side flanker Michael Hooper prevented from scoring a try in the corner by a combined Cooper-Nick Frisby tackle.
Barnes missed with a long range penalty attempt before narrowly missing another one in the final minute. He then scuffed his drop goal attempt well wide, much to his anguish.
The Waratahs didn't help their winning chances with 15 handling errors and five turnovers.
The Reds are likely to be without flying winger Rod Davies for the rest of the competition after he over-stretched his left knee in a tackle and was helped from the field by trainers in the second half.
It was the Reds' 10th win of the season while the Waratahs bowed out with an 8-8 split in Michael Cheika's first season as coach.
Final Score Waratahs 12 (12) Reds 14 (8)
Tries - C.Crawford, B.Barnes
Con - B.McKibbin
Cards - J.Ulugia (25th min yellow)
Tries - B.Lucas
Pen - Q.Cooper 3
Referee: Steve Walsh
Assistant Ref 1:James Leckie
Assistant Ref 2:Ed Martin
TMO : George Ayoub
1. Benn Robinson , 2. John Ulugia , 3. Sekope Kepu , 4. Sitaleki Timani , 5. Kane Douglas , 6. Jed Holloway , 7. Michael Hooper , 8. Dave Dennis (c) , 9. Brendan McKibbin , 10. Bernard Foley , 11. Peter Betham , 12. Berrick Barnes ,13. Rob Horne , 14. Tom Kingston ,15. Cam Crawford
Replacements: 16. Luke Holmes, 17. Jeremy Tilse, 18. Will Skelton , 19. Pat McCutcheon, 20. Mitchell Chapman, 21. Matt Lucas ,22. Ben Volavola
1. Greg Holmes,2. Saia Faingaa,3. James Slipper (c),4. Ed O'Donoghue,5. Rob Simmons,6. Eddie Quirk,7. Liam Gill,8. Jake Schatz,9. Nick Frisby,10. Quade Cooper (vc),11. Luke Morahan,12. Mike Harris,13. Ben Tapuai,14. Rod Davies,15. Ben Lucas
Replacements : 16. Albert Anae,17. Jono Owen,18. Radike Samo,19. Jarrad Butler,20. Beau Robinson,21. Anthony Faingaa,22. Dom Shipperley