in each group, teams played against each other home-and-away in a round-robin format. the group winners and runners-up advanced to the round of 32, where they were joined by the eight third-placed teams of the 2017–18 uefa champions league group stage. the matchdays were 14 september, 28 september, 19 october, 2 november, 23 november, and 7 december 2017.