Former Phoenix favourite, Reddy, who will long be remembered by the Wellington faithful for his penalty shoot-out heroics in a knockout game against Perth two seasons ago, returned to his former stomping ground and was met with jeers from the home crowd.
The 30-year-old was quickly called in to action when he had to make a point-blank save in the opening stages but couldn't deny Ifill only minutes later.
Sydney, who entered the game sitting third on the A-League ladder, were denied a decent shout for a penalty after nearly 40 minutes when Manny Muscat appeared to bring down Scott Jamieson.
Referee Ryan Shepheard waved play on though, despite the protests of the visitors.
Ifill had the chance to double the lead early in the second-half for Wellington, but he blasted his close-range shot over the crossbar.
Reddy then turned villain for Sydney when his mis-hit goal-kick allowed Tim Brown the chance to launch a long-range shot at goal with the keeper off his line and it found the back of the net in the 60th minute.
After Brown's heroics, Ifill was pulled from the field to allow Mirjan Pavlovic to return from a broken arm, but Ifill's performance showed the Phoenix are a different side with him on the paddock.
His good work freed up space for Leo Bertos, who produced his best game of the season down the right flank.
Phoenix skipper Andrew Durante, who played for Sydney on a loan deal in last season's Asian Champion's League, came out fired up against his former team-mates and was busy on defence during the game.
Sydney found their way in to the contest late in the game with a goal from Bruno Cazarine against the run of play in the 70th minute.
Phoenix goalkeeper Tony Warner, who continues to hold his spot ahead of All White Mark Paston, would maybe have wished he did better against the shot of Cazarine, which looked harmless but bounced in to the net.
A busy final 15 minutes ensued and Reddy denied Vince Lia and Chris Greenacre with sharp saves before the final whistle blew.
The result lifted the Phoenix to a share of sixth spot in the A-League with Adelaide United and the Melbourne Heart and snapped a five-game winless streak.
Wellington Phoenix 2 (Paul Ifill 7', Tim Brown 60') Sydney FC 1 (Bruno Cazarine 70') HT: 1-0