His try also saw the Chiefs come away empty-handed and left staring at a six-point gap between themselves and the New Zealand conference-leading Crusaders, with three matches remaining after the international window.
The month off will hardly be a pleasant break for Rennie, coming after a fifth defeat and second on the trot, an ugly pair of results that suggested victory over the Blues before their bye was merely a false dawn.
The Chiefs have struggled for much of the campaign, performing at their best exclusively in patches as the rest of the competition quickly created a blueprint to negate their strengths.
Last season's leading scorers managed just one spell of sustained possession inside the opposition 22 during the first half of today's match - and that came after the hooter.
The Chiefs failed to retain the ball for much of the match, with errors and turnovers as ever-present as they have been all season.
Make no mistake, though, the Waratahs deserve ample credit for their influence on proceedings. Without the ball, they were swarming, aggressive in their line speed and confrontational at the contact area, before tiring as the Chiefs grabbed a brief lead.
The champs' aura of invincibility is well and truly diminished at this point and the Waratahs treated their opposition as if they were the competition's bottom-feeders. And the visitors were equally impressive in attack, easily the more incisive of a pair of powerful backlines, prevented from further points only by the Chiefs' scrambling defence.
The Waratahs enjoyed clear advantages in every attacking statistic and it was surprising to see Israel Folau's try as the sum total of their efforts in the opening half.
Far from rescuing his side's season, Aaron Cruden appeared rusty in his first start in seven weeks, although his clever kick created Aki's first try, a feat repeated by Tom Marshall moments later.
But the lead was short-lived as Dennis and Bernard Foley managed late tries to seal victory, leaving the Chiefs' title defence in real doubt.
Chiefs 17 (Aki 2 tries; Cruden pen, 2 cons)
Waratahs 33 (Folau, Dennis, Foley tries; Foley 3 pens, 3 cons, Beale pen)
HT: 3-13