Shaq Coulthirst made goalkeeper Andre Caio’s life a misery in the first half with a quickfire brace after the stopper parried two shots into his path, while Anton Walkes and substitute Emmanuel Sonupe rounded off a comfortable victory in the second period.
It could have been more, too, with both Andros Townsend and Harry Winks close to hitting the target in the closing minutes of a dominant performance that saw us leapfrog the Portuguese giants into first place in Group B.
We’ll now meet Chelsea, runners-up in Group A, in the quarter-finals in the New Year.
Under-21s Coach Ugo Ehiogu made two changes to the side that drew 1-1 at Everton in the Barclays Under-21 Premier League last week with Tom Glover restored to the side in goal and Josh Onomah coming in on the left side of midfield at Anthony Georgiou's expense.
It was a quiet start to the contest with long-range shots from Porto's Luis Trabulo and Leonardo Ruiz clearing the bar, either side of a clever Townsend run down the right which ended when the recipient of his low ball in, Coulthirst, was thwarted by Diogo Verdasca and goalkeper Caio was able to gather.
Townsend was at it again after a quarter of an hour, collecting the ball deep in our half and running fully 60 yards down the right flank – only for his cross to spin off Antonio Mauricio and into the arms of Caio.
The Porto custodian was certainly seeing more of the ball than his opposite number as the half drifted past the 20-minute mark and we began to pin the visitors in.
Porto nearly found a breakthrough on the counter-attack on 27 minutes but, after Claudio Ribeiro shrugged off Luke Amos, Winks came across to block.
Townsend had a strong appeal for a penalty waved away by referee Josh Smith within 60 seconds after taking a tumble under the challenge of Verdasca, but it mattered little as we took the lead on 31 minutes.
Again Townsend was involved as he fed Onomah, with the midfielder running into bodies centrally before trying his luck with his right foot. He was leaning back as he shot but still Caio couldn't hold his effort and parried it, with Coulthirst racing in to smash home the rebound through the legs of the goalkeeper from a tight angle on the right.
Four minutes later it was 2-0 and again Caio was left kicking himself as he palmed Anton Walkes' angled drive down in the six-yard box with Coulthirst close at hand to gratefully sweep home the loose ball.
The visitors didn't really threaten for the remainder of the first period, although Cameron Carter-Vickers had to stay strong to block a fierce effort from Ruiz six minutes before the break.
A few seconds later, his centre-back partner Milos Veljkovic clipped an inch-perfect ball over the top for Winks, who controlled on his chest in the area after the ball's initial bounce, but he couldn't quite keep it under full control as it drifted away to the right and he ended up thumping a shot across the face of goal but marginally wide of the far post.
The second half was comfortable for our lads with Glover again largely a spectator. Ruiz had an early claim for hand-ball against Carter-Vickers turned down but it was fairly quiet in terms of chances until the last 20 minutes.
Coulthirst was denied his hat-trick by Caio at his near post before the keeper made a fine save on 72 minutes, diving low to his left to keep out Winks’ effort through a crowd of bodies in the area after Townsend played a free-kick short to his feet.
Ruiz fired wastefully over the bar from 20 yards after pouncing on a loose ball in midfield before we extended our lead on 77 minutes. Will Miller split the Porto back line with a through ball and while fellow substitute Shayon Harrison realised he would be flagged off-side and thus held his run, Townsend went storming forward and squared for Walkes to tap in from close range.
Miller himself then had an effort charged down but he was involved again on 89 minutes as we wrapped up the scoring, sending Sonupe through in the right channel to roll the ball past the advancing goalkeeper and into the net.
There was still time for Townsend and Winks to have a go – the England international winger twisted and turned his way into the box but curled just wide in the 90th minute, while Winks’ effort was gathered by Caio at the second time of asking in what was the last action of the game.
Spurs U21s: Glover, Walker-Peters, Amos, Lesniak, Carter-Vickers, Veljkovic (c), Townsend, Winks, Coulthirst (Harrison 71), Walkes (Sonupe 77), Onomah (Miller 46). Substitutes (not used): Georgiou, McDermott (GK).
Porto U21s: Caio (c), Mauricio, Trabulo, Soares, G Moreira (Pedro 75), Ruiz, R Moreira, C Ribeiro, Varela (Santos 58), S Ribeiro (Cassama 58), Verdasca. Substitutes (not used): Fernandes, Fonseca, Mata, Costa (GK).