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FC Basel 1-1 Steaua Bucharest
Giovanni Sio scored a late equaliser in second-half stoppage time for Basel, who escaped with a draw at home to Steaua.
Steaua had taken the lead in the 17th minute through Federico Piovaccari.
Basel are third in Group E with five points. Steaua remain fourth with two points.
Napoli 3-2 Marseille
Napoli kept pace with Arsenal in Group F with a thrilling 3-2 win at home to Marseille.
Gonzalo Higuain scored twice, including the 75th-minute winner. Marseille took an early lead through Andre Ayew, only for Gokhan Inler to equalise with a sweet volley in the 22nd minute.
Higuain's first goal put Napoli ahead 2-1 in the 24th minute, but Florian Thauvin pegged back the hosts midway through the second half.
With the win, Napoli moved onto nine points, level with Arsenal atop the group. Marseille still have not claimed a point and have been eliminated from contention for the knockout phase.
Zenit St. Petersburg 1-1 Porto
Hulk scored a goal and missed a penalty as Zenit drew with Porto in Wednesday's early kickoff.
The Brazilian forward, who formerly played for Porto, canceled out Lucho Gonalez's opener with a 28th-minute strike. But his weak penalty was saved in the 52nd minute as Porto held on for a point.
Zenit are second in Group G with five points, one ahead of Porto.
Atletico Madrid 4-0 Austria Vienna
Atletico booked passage to the knockout stage with an easy 4-0 win at home to Austria Vienna in Group G.
Miranda, Raul Garcia, Filipe Luis and Diego Costa scored the goals for Atleti. Costa missed a penalty in the second half.
Atletico lead Group G with a maximum 12 points from four matches. Austria Vienna have one point.
Ajax 1-0 Celtic
Lasse Schone scored the game's only goal in the 51st minute as Ajax claimed their first win of the group stage.
The Dutch side are now third in Group H with four points, just one behind second-placed AC Milan. Celtic are fourth with three points, having beaten Ajax in the return fixture two weeks ago.