A Georgios Samaras header has put Celtic on course to qualify for the Champions League knockout stages following the first 45 minutes against Benfica.
The hosts made a perfect start to the match by taking the lead in the sixth minute when John fired the ball low past Fraser Forster after Celtic had failed to clear the danger inside their own box.
Benfica controlled possession following the goal as Celtic failed to assert themselves in midfield at the Estadio da Luz.
Oscar Cardozo wasted a chance to double Benfica's lead when he drilled his shot wide of the target after Eduardo Salvio had found the striker in space.
Celtic punished their opponents for failing to build on their lead in the 31st minute, with an unmarked Georgios Samaras heading in a Charlie Mulgrew corner from close range.
Benfica were shocked by the equaliser and Samaras went close from 25 yards as the visitors finished the first half as the better side.
However, the home side went close to taking the lead again on the stroke of half-time with Forster being forced into a good save by John's strike from the corner of the box.
