The Force's shock victory was achieved despite losing Wallabies prop Pek Cowan when he was taken off on a medicab after having his head and neck stabilised following a collapsed scrum, midway through the first half.
The Force clinched the win with two tries in the final quarter to winger Luke Morahan and Cowan's replacement, Francois van Wyk.
A 68th minute cross-field kick from halfback Alby Mathewson found Morahan in space, and though he slipped over, the winger had time to regain his footing and collect the ball and score.
The win was sealed five minutes later when van Wyk caught an attempted chip and chase kick from Kurtley Beale inside the Waratahs' 22.
Waratahs winger Rob Horne narrowed the gap with his second try with just under four minutes to go.
His first well finished try in limited space in the 54th minute, came just seconds after Mathewson had been sin-binned.
The Force also made the running in the first half, which ended with the visitors holding an 8-3 lead.
Former Waratahs flanker Chris Alcock crashed over from a rolling maul for the only try of the half, shortly after Wycliff Palu had been sin-binned.
Fellow flanker Angus Cottrell smashed through attempted tackles from Dave Dennis and Wycliff Palu in the 46th minute.
Horne's first try gave the Tahs brief hope, but their inability to sustain pressure meant they were no match for a more disciplined Force side.
- AAP
