The Shots hitman followed up his late, late winner in the 1-0 victory last October with a carbon copy header in the very same net, thus putting paid to a gutsy Gravesend fightback.
Sills climbed highest to nod home Darren Barnard's 92nd minute free-kick to the huge relief of the Recreation Ground — this following another unconvincing display from the Shots, who failed to combat ten men for over an hour and almost ended up throwing away the advantage.
The Shots looked odds on for victory when former Shot Lee Protheroe was sent off on the half hour mark for handling the ball on the line, denying Matt Somner a certain goal.
When Barnard rifled home the resulting penalty to make it 2-1 to the Shots, you would have put your mortgage on the hosts cruising to an easy win.
But somehow they lost their way and allowed the ten men back into the match, only for Sills to deny them at the death.
A disappointed but relieved boss Terry Brown said: "It was a very poor second half performance. When you go in at half time 2-1 up and playing against 10 men, you would want to click into gear and bury the opposition. We certainly didn't do that today and credit must go to Gravesend who played their football, kept their shape and deserved something out of the game.
"We have Tim Sills to thank for three points today. He did it against Gravesend last year and I was as grateful then as I am now.
"They will probably point to the late free-kick as the difference between the teams today but I would say the difference was us having a striker of the calibre of Tim Sills."
Thankfully for Brown, goal ace Sills won his fitness battle against a sprained shoulder sustained in last week's FA Cup clash at Burnham, and it took him just four minutes to prove his worth.
Good work down the right from Ryan Scott and Somner allowed the latter to float in an inch-perfect cross from the by-line which Sills buried with typical aplomb at the near post.
The goal was reward for a bright start by the Shots and had inconsistent referee Rob Lee spotted a blatant hand ball in the Gravesend wall in preventing Barnard's goalbound free-kick, the Shots would have had an ideal chance to double the lead from the spot.
As it was, however, it was the visitors who were handed a lifeline nine minutes later when Bradley Johnson's driven cross was alleged to have struck the hand of Somner, leaving striker Charlie MacDonald to slot home the resulting penalty and restore parity against the run of play.
Ten minutes later, however, Gravesend were silenced.
Experienced new signing Brian Tinnion floated in a corner from the left, Somner climbed highest to head goalbound, only for Protheroe to stick out a hand on the line to prevent him.
By the letter of the law, Protheroe had to walk back down the tunnel he knows so well and Barnard added to the Fleet misery by dispatching the spot kick expertly into the top corner of the net.
Gary Holloway squandered an excellent chance at the near post two minutes before the break and it was a miss which was to prove costly as the Shots suddenly lost their way, causing Gravesend to find heart after the break.
MacDonald shot wide after Bobby Bowry pinged in a cross from the left, while Onome Sodje brought a fine double save by Nikki Bull after being put clean through by Jay Saunders.
Brown responded by making a rather rash double substitution on the hour, bringing on Andre Boucaud and Hassan Sulaiman in place of Holloway and Ahmed Deen.
That however, simply made things worse as the Shots lost their shape completely and Gravesend, despite their numerical disadvantage, piled on the pressure.
Indeed, with four minutes remaining, youngster Luke Moore floated in a cross from the left and MacDonald beat a stranded Bull with a looping near post flick.
Despite admitting his mistake in the substitution later, Brown's anger and frustration was for all to see, but somehow the Shots found an extra gear.
A bandaged Scott just failed to connect with a diving header from Barnard's driven centre, while young substitute Luke Walker, brought in from a loan spell at Camberley Town, was denied a one-on-one with Fleet's former Farnborough keeper Craig Holloway, courtesy of excellent cover work by Scott Gooding.
But with the seconds ticking down, the Shots were handed a lifeline.
Mr Lee adjudged Fleet full back Ross Smith to have impeded Sulaiman down the left flank much to the annoyance of the Gravesend bench. Barnard floated in the free-kick with his usual splendour and Sills did the rest with a header which looped over Craig Holloway and into the roof of the net.
The relief from the Rec was for all to see as Sills wheeled away in delight having rescued his side once again.
Performance aside, this will go down as a massive boost to boss Brown, who is slowly starting to get rewards from a more settled squad.
Indeed, this was the first time this season the boss had picked a side without any loan players starting, and once Sills gets a partner to ease the goalscoring burden, the squad, on paper, may even look good enough to mount a late Play-Off challenge.
SHOTS: Bull 7, Somner 7, Brough 6, Heald 7, Barnard 8, Crittenden 6 (sub Walker 85mins), Holloway 6 (Sulaiman 60mins, 5), Deen 6 (Boucaud 60min, 6), Scott 7, SILLS 8, Tinnion 7. Not used: Jinadu, Weait. Att 2415, Ref R Lee 4.