The Waratahs beat the Rebels 38-32 in their Super Rugby AU clash in Sydney on Saturday, but the margin of victory was probably not enough to ensure the home side qualify for the playoffs of the domestic competition.
Rebels captain Matt Toomua kicked a late penalty to give his side a vital bonus point which should be a factor in determining the composition of the top-three placings.
The Waratahs have a bye in next Saturday's final round of the regular season and have finished on 19 points, while the Rebels, who are on 15 points, could finish third if they beat the winless Western Force next Saturday.
The Brumbies sealed top spot on Friday with a 31-14 win over the also winless Force in Canberra to move to 28 points.
The Queensland Reds are second on 21 points and face the Brumbies in Brisbane next Saturday.
The Waratahs showed their intentions to secure a bonus point with a willingness to keep the ball alive and were rewarded with a 14-3 lead through tries to scrum-half Jake Gordon and number eight Jack Dempsey.
Toomua, who slotted an early penalty, also added a converted try for the Rebels when Waratahs lock Ned Hannigan was given a yellow card following a succession of tryline penalties.
Fly-half Will Harrison kicked an injury-time penalty to give the Waratahs a 17-10 lead at halftime.
Waratahs prop Harry Johnson Holmes, centre Joey Walton and Harrison all crossed in the second half, with the fly-half's try with four minutes remaining taking them to 38-29 lead.
Rebels No 8 Isi Naisarani crossed twice in the second half, while Marika Koroibete also scored before Toomua nailed the 79th-minute penalty to ensure the competition would not be decided until next week.