The forward was lethal in the penalty box, producing two fine left-footed finishes either side of a rare Milos Veljkovic goal as we surged into a 3-0 lead in the Barclays Under-21 Premier League clash at Aldershot Town FC.
Tammy Abraham's late header was a mere consolation for the Blues, who only really found their attacking impetus after our third goal.
Under-21s Coach Ugo Ehiogu made one change to the side that drew with Liverpool last week, with the more experienced Kenny McEvoy replacing Keanan Bennetts on the right of midfield – and it was down that side that we applied some early pressure.
McEvoy – twice – and Kyle Walker-Peters saw crosses repelled from the right wing position in the opening eight minutes but, with both sides eager to get on the ball and press forward, possession was frequently turned over and chances were at a premium.
Chelsea's only real early threats also came from their right, with Charly Musonda's eighth-minute ball in tamely headed wide by Jay Dasilva, while his second cross seven minutes later was headed upwards by Walker-Peters with goalkeeper Tom Glover gathering the falling ball well under pressure.
Both sides traded headed chances as the half progressed, Cameron Carter-Vickers' effort from Anton Walkes' corner being comfortably caught by Mitchell Beeney in the Chelsea goal on 23 minutes, before Jake Clarke-Salter's powerful attempt from Musonda's flag kick flew just over the bar a few minutes later.
We found the breakthrough on 28 minutes and it came following an incisive move. Zenon Stylianides played the perfect pass between right-back Ola Aina and centre-back Fikayo Tomori for Filip Lesniak to go clean through. He was well placed to shoot, but unselfishly squared for Harrison to sweep low beyond Beeney.
The Blues nearly produced a response on 35 minutes when Fankaty Dabo dangerously whipped a low ball in from the left, but Veljkovic was there to intervene at the vital moment.
Chelsea skipper Charlie Colkett ballooned a free-kick over the top shortly afterwards, while we saw a good opportunity go begging three minutes before the break when Walkes caught Dabo off-balance and sent McEvoy racing towards the near post from the right side. Unfortunately for the winger, his low driven ball was blocked by the legs of Beeney and we went in at the break a goal to the good.
The first half may have been short on clear chances, but the game exploded into life five minutes after the restart with action at both ends.
First the lively Musonda fashioned a shooting chance out of nothing but saw his right-footed rasper arrow over the top, before Anthony Georgiou floated over a cross from our left which dropped perfectly into the path of McEvoy at the back post. Somehow, though, the winger failed to steer the ball home as he threw his body at it.
Unfazed, we recycled the attack and Stylianides saw his low shot take a slight touch off a defender as it drifted agonisingly wide of the far post.
We finally made our pressure count from the resulting corner, though, as Walkes' flag kick took a touch off a Chelsea head and fell kindly for Veljkovic to slide home at the back post with 52 minutes gone.
Clearly boosted by the second goal, we went in search of a third with Georgiou playing in Walkes on 58 minutes, but Clarke-Salter got his body between him and the ball and was able to shepherd it clear.
A minute later we did find the target and it was Harrison once more who steered home at the first time of asking with his left foot after a fine cross from the left by Georgiou.
Chelsea were seemingly stung into action after that – with substitute Miro Muheim leading the charge.
His first effort rolled comfortably into the grasp of Glover, while he dragged his second shot wide a few minutes later after chipping the ball beyond Carter-Vickers.
Harrison, in search of his hat-trick, tried a low shot from outside the box that didn't cause too much trouble for Beeney on 73 minutes, while Muheim played in fellow substitute Kyle Scott at the other end, but Carter-Vickers was quick to close him down and the Chelsea man's effort from a very tight angle was smothered by Glover.
Chelsea came at us on the counter a handful of times as they enjoyed their best spell of the game, while Abraham stung the palms of Glover after cutting in from the left on 80 minutes.
It was Abraham who pulled a goal back for the Blues with three minutes of normal time left, powerfully nodding a right-sided cross from Aina down into the net.
We had to stay composed defensively in the closing minutes to prevent Chelsea from making further inroads, although the last chance of the game went to us in the fourth of five additional minutes. Substitute Ryan Loft found his path blocked by Tomori as he broke forward, so he opted to play the ball against the Chelsea defender and chased the loose ball. Tomori was able to recover well enough to put the striker off his shot, though, with the ball deflecting harmlessly behind.
Chelsea U21s: Beeney, Aina, Tomori, Clarke-Salter, Dabo, Colkett (c), Musonda, Sammut (Scott 54), Abraham, Mitchell (Muheim 59), J Dasilva (Maddox 54). Substitutes (not used): Conroy, Collins (GK).
Spurs U21s: Glover, Walker-Peters, Amos (Bennetts 74), Lesniak, Carter-Vickers, Veljkovic (c), McEvoy (Maghoma 82), Stylianides (Loft 67), Harrison, Walkes, Georgiou. Substitute (not used): Voss (GK).