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A brilliant goal from Sutton's Tom Bolarinwa sunk Ebbsfleet at Stonebridge Road on Saturday.
Bolarinwa settled a tight battle of two promotion rivals in front of the biggest attendance for a league game at the ground since the KEH takeover in 2013.
The home side lacked real spark, created little in front of goal and defeat saw them drop out of the National League play-off places.
But with a game in hand over seventh-placed Dover, their fate is still in their own hands ahead of Tuesday's long trip to Gateshead.
Ebbsfleet came into the game unbeaten in 11 games and Jack Powell replaced Luke Coulson in manager Daryl McMahon's only change.
The players walked out to a cacophony of noise from packed stands on Fleet's Community Day but it was Sutton who bossed the first half.
Quick to close down and dangerous whenever they went forward, Paul Doswell's men certainly didn't look like a side who had lost four of their previous five matches.
Bolarinwa and Kieron Cadogan were able to find space on the flanks and only their poor delivery let Fleet off the hook.
But a shot from Tommy Wright rattled the post as Sutton stayed on the attack.
Ebbsfleet looked wobbly in defence and Louis John's deep diagonal ball forced Dave Winfield to concede a corner before Cadogan set up Wright for a shot which fizzed wide.
At the other end, Corey Whitely's half-volley was straight at goalkeeper Jamie Butler but it was far too easy for the U's to work the ball into good areas when they went forward.
Positive play by Chris Bush carried him to the corner of the penalty area and drew a foul from Nicky Bailey, who was shown a yellow card. Powell took the free-kick and swirled an ambitious effort just beyond the angle although Butler probably had it covered.
Ashmore saved well from Cadogan at his near post after Ebbsfleet failed to clear a corner.
Dean Rance was then caught very late by Crag Eastmond, who followed Bailey into the book, and Rance was lucky to carry on unscathed.
But still the best chances were falling to Sutton. Cadogan slashed a shot wide after Dave Winfield sliced a clearance and Ashmore was forced to make a double save from Josh Taylor.
Jamie Collins did well to turn Whitely's dangerous cross over his own crossbar and Whitely powered a shot wide from distance.
Whitely's next effort deflected behind for a corner, which Winfield headed straight at Butler. Sutton almost scored on the counter-attack as Ashmore denied Bolarinwa one-on-one.
Ebbsfleet lost Sean Shields to injury just before half-time and took Andy Drury off at the interval, with Myles Weston and Coulson the men coming on.
The hosts had missed Coulson's dynamism in the first half and they came out for the second looking more purposeful.
Coulson drilled a low shot straight at Butler although Lawrie Wilson had to be on his toes to cut short Aswad Thomas' marauding run into the penalty area at the other end.
Neither side was giving an inch with so much at stake in the race for the play-offs. Play became increasingly physical with Sutton slowing things down at every opportunity.
It was going to take something special to break the deadlock and that something arrived 13 minutes from time.
Bolarinwa showed great balance on a mazy run which took him through the Fleet defence before rifling a shot inside Ashmore's near post.
On came Norman Wabo for the last 10 minutes but another substitute, Ross Lafayette, almost punished Kenny Clark's hesitancy with a second goal for Sutton. Ashmore eventually rescued his centre-half.
Fleet pumped the ball forward late on but lacked quality and Sutton held on with no alarms.
Ebbsfleet: Ashmore, Wilson (Wabo 79mins), Winfield, Clark, Bush, Rance, Drury (Coulson 46mins), Whitely, Powell, Shields (Weston 45mins), Kedwell. Subs not used: McCoy, Miles.
Sutton: Butler, John, T Thomas, Collins, A Thomas, Bailey, Eastmond, Taylor, Bolarinwa, Cadogan (Lafayette 76mins), Wright (Beautyman 79mins). Subs not used: Beckwith, Stearn, Harrison.
Attendance: 2,852.