Tottenham Hotspur suffered their first loss in Group D after Sporting Lisbon's late show condemned Antonio Conte's troops to a 2-0 defeat at Estadio Jose Alvalade.
Spurs retained the front trio of Richarlison, Son Heung-Min and Harry Kane while Sporting centre-forward Marcus Edwards started against his former side.
The Portuguese side threatened in the opening exchanges after Pote had a sight of goal and unleashed a strike that forced goalkeeper Hugo Lloris into an out-stretched save.
In a very tight game, Richarlison had a goal for Spurs disallowed for offside just before the half-time break.
Antonio Conte's charges enjoyed a strong start in the second stanza as Kane came to life with a fierce left-footed effort that was pushed away by goalkeeper Antonio Adan.
However, the game was on the edge in the dying minutes and Paulinho broke the deadlock for Sporting in the last minute with towering header from a corner-kick.
Sporting put the game to bed in stoppage time courtesy of a strike by Arthur Gomes from a solo-run, as they clinched back-to-back wins.
Meanwhile, Inter Milan got off the mark in Group C after beating ten-man Viktoria Plzen 2-0 on Tuesday evening at Doosan Arena.
Inter rotated the side that edged Torino in Serie A, as the likes of goalkeeper Andre Onana, Henrikh Mkhitaryan, Joaquin Correa were brought in.
Simone Inzaghi's charges opened the scoring on 20 minutes after Edin Dzeko found space from a through ball by Correa and fired a low strike past shot-stopper Jindrich Stanek.
Plzen struggled to get into the game, with Inter dictating terms but the Czech outfit only threatened towards the end of the first stanza.
The game opened up in the second stanza but the hosts were reduced to ten men on the hour-mark after Pavel Bucha had a yellow card upgraded to red for a studs up challenge after a Video Assistant Referee (VAR) review.
With Plzen vulnerable on counter-attacks, Denzel Dumfries eventually doubled the lead for Inter 19 minutes before time with a strike towards the far corner after a through ball by Dzeko.