Borussia Dortmund, last season's runner-up, is in danger of missing the knockout stage of the Champions League after losing 1-0 at home to Arsenal on Wednesday.
Aaron Ramsey's goal with Arsenal's first attempt on goal in the 62nd minute ended Dortmund's run of seven consecutive home wins and dropped Juergen Klopp's team to third place in Group F with six points from four matches.
Arsenal and Napoli have nine points each, while Marseille is without points after losing 3-2 at Napoli. Dortmund, which lost at Napoli, must beat the Italian club at home to preserve its chances of advancing.
''We still have two matches and everything is possible,'' said Klopp, who returned to the sidelines after a two-match suspension.
''We created much more today up front and we started well in the second half. But they scored with their first chance of the match and we could not come back. This defeat could have been avoided.''
Dortmund had the better chances until Ramsey's goal, but Arsenal was the more dangerous lineup once it had gone in the front. The Premier League leaders have stretched their run to 15 consecutive wins on the road in all competitions.
Arsenal broke the deadlock when Mesut Ozil crossed into the area, Olivier Giroud nodded the ball into the path of Ramsey and the midfielder got ahead of Neven Subotic to head past Roman Weidenfeller.
''Dortmund was the better side in the first half. But after our goal we should have scored another,'' Arsenal's coach Arsene Wenger said. ''Once we were in the lead, we were not so much under pressure.''
Dortmund appealed for a penalty in stoppage time were ignored after defender Per Mertesacker pulled down striker Robert Lewandowski.
Dortmund's best chance of the first half came in the 37th but Henrikh Mkhitaryan sent his shot wide when clean through with just Arsenal keeper Wojciech Szczesny to beat. Earlier, Subotic had volleyed wide and Lewandowski also had an effort off the target.
Both teams closed the space and gave little room, although the match had pace.
Szczesny did well to turn away a header by Marco Reus early in the second half, but then Arsenal stunned the home side with Ramsey's goal.
Ramsey nearly doubled the lead two minutes later, but Weidenfeller blocked his point-blank shot. From the corner, Nuri Sahin cleared off the line from Giroud.
A glancing header by Mertesacker went over in the 70th as Arsenal piled on the pressure.
