Huddersfield survived a last-second scare to pile on the agony for Leeds with a fourth successive win at Headingley to deny the Rhinos a chance to return to the top of the First Utility Super League.
Leeds thought they had snatched a victory when England winger Tom Briscoe won the race to the line to touch down Joel Moon's speculative kick after the final hooter had sounded but video referee Phil Bentham disallowed the score for obstruction, leaving the visitors to celebrate a nailbiting 24-22 success.
It was a third defeat in four league games for Leeds and their first at home this year but the Giants are starting to look the part after claiming a seventh win in their last eight matches.
Leeds, who have not beaten their Yorkshire rivals in seven matches, were missing eight regulars through injury and suspension but they fought to the last in a dramatic clash.
There was little hint of the drama to come when England centre Kallum Watkins showed his class, taking Brett Delaney's pass, and slipping the attempted tackle of Joe Wardle to open the scoring on five minutes.
Leeds' defence - the best in the league - was also made to look average three minutes later when Huddersfield scrum-half Kyle Wood went through a gap left by Ryan Bailey and Stevie Ward to level the scores with Danny Brough's conversion.
The Rhinos re-took the lead when the visiting defenders could not put centre Moon to ground and full-back Ben Jones-Bishop, deputising for the suspended Zak Hardaker, took a return pass to score their second try.
Sinfield's second goal made it 12-6 but both defences tightened considerably in a scoreless second quarter, with Huddersfield ruining some promising build-up work with a series of careless fumbles.
Second rower Brett Ferres failed to make use of a two-man overlap as he was held up over the line and Larne Patrick and Jermaine McGillvary were both brought down agonisingly short as the Giants strove to get level.
The equalising score eventually came seven minutes into the second half when Huddersfield pounced on a handling error deep in his own half by Jones-Bishop.
Ferres and Brough worked the ball from one side of the field to the other and McGillvary took Aaron Murphy's pass to go over at the corner.
Brough's touchline conversion made it 12-12 and the Giants completed the turnaround by adding two further quickfire tries in contrasting fashion to storm into a 12-point lead.
Prop Eorl Crabtree forced his way over from close range and Wardle went in from long range following a clean break by Ferres.
Brough added both conversions but Leeds struck back with two tries in three minutes to put the game on a knife edge.
Prop Bailey powered his way past three defenders to touch down wide out and stand-off Liam Sutcliffe grabbed an opportunist try after the ball bounced loose from Sinfield's kick on the last tackle.
Sinfield could land only one of the goals, though, to leave his side trailing by two points.
Huddersfield thought they had clinched the win when Brough's long pass got winger Jodie Broughton over at the corner but video referee Phil Bentham disallowed the try for an obstruction in the build-up.