Beşiktaş's stunning victory at Napoli, who had won both their opening fixtures, has put the Turkish side within a point of the Group B leaders.
In the first competitive meeting between the teams, Beşiktaş twice let the lead slip before Vincent Aboubakar struck a late winner, his second goal of the game, to secure a 3-2 victory.
Adriano Correia Claro had opened the scoring in the 12th minute for Beşiktaş but Napoli's Dries Mertens scored in the 30th minute to equalize.
A Manolo Gabbiadini penalty in the 69th minute, with Lorenzo Insigne having earlier missed from the spot, kept Napoli in the hunt before Aboubakar gave the Turkish Super League champions their first victory over Italian rivals in 12 tries.