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Dartford scored a 94th-minute equaliser on Saturday to extend their unbeaten run in National League South to 10 matches.
Tom Bradbrook came off the bench to convert Ryan Hayes' corner in the dying seconds of stoppage-time after Billy Bricknell had put visitors Chelmsford in front.
The draw moves sixth-placed Dartford level on points with Oxford City and Gosport in the race for the play-offs.
Callum McNaughton returned at centre-half after missing Monday's 1-1 draw with Margate, with Tom Wynter moving forward to replace the injured Lee Noble in Dartford's midfield.
Andy Pugh was fit enough for a bench which also featured fellow strikers Tom Bradbrook and Ben Francis but full-back Lee Burns was missing with a groin injury.
For the visitors, defender Rory McAuley and winger Luke Daley returned to Princes Park having left Dartford in the summer while former Ebbsfleet striker Bricknell led the City attack.
After surviving several of Bagasan Graham's long throws in the opening exchanges, Dartford played almost all the football during the first half.
They started with two holding midfielders but manager Tony Burman quickly pushed Ebou Adams further forward to support Elliot Bradbrook as the hosts dominated possession.
McNaughton headed over from a Ryan Hayes corner and Sykes dragged a shot across the face of goal after Danny Harris had dummied a Tom Gardiner throw-in.
Another Hayes corner was nodded over by Ronnie Vint, although the whistle had already gone for a foul.
Adams snatched a shot well wide from 30 yards and then showed great persistence to dispossess Graham as he was trying to let the ball run out for a goal kick. Adams kept the ball in on the byline and crossed low but it wouldn't fall for anyone in a white shirt.
A rare Chelmsford attack saw Graham's header from a Lee Sawyer cross drift well wide, before Sykes latched onto Vint's long ball out of defence and shoot straight at goalkeeper Tim Brown.
Dartford gave the ball away at the other end and Ben Marlow forced Deren Ibrahim into his first save of the afternoon although the hosts had the final chance of the first 45 minutes.
Hayes, having swapped flanks with Harris, picked up possession on the left-hand side and stung Brown's palms with a rising drive from just outside the box.
Dartford continued in a similar vein at the start of the second half. Adams pulled a shot wide from distance before Bradbrook burst through midfield and shot low at Brown, who gathered on the bounce.
Hayes delivered a deep cross from the right and it ran all the way for Harris at the back post. His snap-shot was turned round the post by Brown, who then did superbly to keep out Harris' close-range effort from the subsequent Hayes corner.
A long throw from Graham flashed narrowly wide after glancing off a Dartford head but still it was the home side doing most of the pressing and Chelmsford looking to strike on the break.
Burman sent Pugh on for the last 25 minutes and the substitute almost made an immediate impact. He collected Hayes' pass and forced a scrambling save from Brown when his goalbound effort took a deflection inside the box.
But the visitors came so close to breaking the deadlock when Billy Hunt, bearing down on the penalty area, beat Ibrahim with a ferocious shot with rattled the crossbar and bounced to safety.
With play now stretched, Pugh turned neatly and fired a shot over before Adams saw his curling strike deflect just wide after shifting the ball onto his left foot.
But the Clarets hit the front 10 minutes from time when Dexter Peter flicked on a cross from the left and Bricknell guided a header inside Ibrahim's left-hand post.
That looked like being the winner until Tom Bradbrook applied the finishing touch to Hayes' corner in the dying seconds to send the home faithful wild.
Dartford: Ibrahim, Gardiner, Onyemah, Vint, McNaughton, Adams, Hayes, E Bradbrook, Sykes (Pugh 65mins), Wynter (Cogan 82mins), Harris (T Bradbrook 87mins). Subs not used: Golding, Francis.
Chelmsford: Brown, Girdlestone, Graham, Sawyer, Ekpiteta, McAuley, Marlow, Hunt (Morgan 82mins), Bricknell (Green 90mins), Peter, Daley. Subs not used: Gordon, Pearce.
Attendance: 1,205.