Red Star Belgrade secured their first-ever Champions League group stage win after two goals from towering striker Milan Pavkov helped the Serbian side to a memorable 2-0 home victory over Liverpool.
The result left Liverpool on six points from four games in Group C and facing a tricky path to the knockout stages after the setback in the cauldron of Red Star's Rajko Mitic stadium.
Napoli forward Lorenzo Insigne converted a second-half penalty to cancel out Juan Bernat's goal in first-half stoppage time for Paris St Germain in a tense 1-1 draw which left the group wide open.
PSG took control in the first half and were rewarded when Kylian Mbappe set up Bernat who scored from close range as the Napoli defence was caught napping just before the break.
Insigne levelled from the spot in the 63rd minute after Jose Callejon pounced on a mistake by PSG defender Thiago Silva and was upended in the area.
Liverpool and Napoli have six points from four games in Group C with PSG on five and Red Star Belgrade on four.
Liverpool play Napoli and PSG in the final two rounds.
Barcelona qualified for the knockout stage after a 1-1 draw at Inter Milan.
Barca were still without talismanic figure Lionel Messi through injury but were the better side for most of a game that was high on intensity but lacking in quality until an electrifying final 10 minutes at a rocking San Siro.
Barca top Group B with 10 points after four games, Inter are second with seven while Tottenham Hotspur are third on four and PSV Eindhoven are bottom and out of the competition with one.
Spurs came from behind with a goal to Harry Kane and an own goal to beat PSV 2-1 in London.