Rodrigo Bentancur's equaliser saved Tottenham Hotspur from a third successive defeat in all competitions.
Sporting punished Antonio Conte's side for a poor first-half display that brought mocking remarks from the home side.
"I think it's clear to see we want to play as we did in the second half for 90 minutes. We know we're capable of doing it, but it's what we're struggling to do."
Those were the words of Spurs star Matt Doherty, who has admitted Tottenham are struggling to be consistent at a high level throughout a game.
The Irish right-back was disappointed Antonio Conte's Lilywhites could not secure a victory over Sporting Lisbon to book their place in the next round of the Champions League.
The north London club started the match on Wednesday night poorly, while the Portuguese side were on the front foot and got their reward when ex-Spurs prodigy Marcus Edwards found the net with a brilliant low shot in the 22nd minute, and the set-back brought loud jeers from the home fans.
Spurs improved their performance in the second half and had to thank keeper Hugo Lloris who made a crucial save as he prevented Sporting substitute Flávio Nazinho from hitting the net. Moments later the left-back had a chance to make it 2-0, but he shot wide with the goal at his mercy.
However, Spurs huffed and puffed and eventually took advantage as Rodrigo Bentancur leaped high and headed home from a corner to make it 1-1.
England defender Eric Dier also had a chance to put the game to bed but couldn't ensure a comeback win, and the moment came when the Tottenham Hotspur Stadium was sent into ecstasy as Harry Kane latched onto Brazil defender Emerson Royal's knockdown, firing the ball into the far corner.
Spurs thought they had scored a dramatic stoppage-time winner but following a perennial VAR check, Kane's goal was ruled out for offside, and the home fans went from ecstasy to despair, with Conte seeing a red card for his antics.
The result at the Tottenham Hotspur stadium means it is going to be a winner-takes-all Group D showdown with Marseille on the final matchday next week, although, the Premier League club only need an away draw against the Ligue 1 club to reach the knockout phase.
'I'll have to look at the rule book.'
"I think you can see from the celebrations we thought we won it. Pfffft. I don't really know what happened at the end," the Tottenham defender Doherty told BT Sport after the intense match.
"I thought because it went backwards and hit a defender it was a different phase of play. I'll have to look at the rule book. A few of us don't have a clue what happened.
"If we're being honest we didn't play that well. They nullified us in the first half and deserved to be ahead.
"We're top of the group. It's not how we wanted the night to go but it's still in our hands.
"I wish we knew [why they can't play well for a whole game]. We're analysing games and training sessions, and watching a lot of videos to find out how we can perform for 90 minutes and play like we can play.
"We have a few days to look back and see what went wrong before we play on Saturday.
"I think it's clear to see we want to play like we did in the second half for 90 minutes. We know we're capable of doing it but it's what we're struggling to do."