Denly hit an unbeaten 76 from 60 balls to steer his side to their victory target of 151 at the Adelaide Oval and jump them above the Strikers in the table.
It was a major upturn in form from the Kent all-rounder, whose previous high score in six innings of this year's Big Bash League was just 14.
Having been set their target of 151, Sydney lost opener Justin Avendano for a duck before Denly steadied the ship alongside Daniel Hughes.
But Denly was far from his fluent best, making a run-a-ball 31 at the halfway stage of the innings, with Adelaide seemingly in control - Sydney at 67 for two having lost Hughes for 24.
The 33-year-old, who featured in England's T20 series in Sri Lanka, was then handed a reprieve as Cameron Valente dropped a tough caught-and-bowled chance when Denly skied one.
That let-off appeared to free Denly up and he began to up his strike rate, hitting Liam O'Connor for two fours and a six with the first three deliveries of the 12th over.
Denly's half-century came from 39 balls but he lost two partners in the next over. Rashid Khan, who played for Sussex in the T20 Blast in 2018 and is set to do so again in 2019, dismissed Moises Henriques and Jordan Silk in the same over.
But Denly carried on, and was ably supported by 21-year-old Australian Josh Philippe with a blistering 30 from just 15 balls as Sydney secured their third away victory of the competition.
Fellow England international Tom Curran claimed a solitary wicket in the first innings, finishing with one for 38 from his four overs, and briefly became the leading wicket-taker in the competition before being overtaken by team-mate Steve O'Keefe.
The Sixers are up to second in the standings, albeit having played two games more than some sides, including leaders Hobart Hurricanes - who have a perfect record from their five matches.
