The visitors got off to a dream start in the first minute when Keita ran down the left wing and squeezed an effort beyond the reach of Ivan Stoyanov.
The midfielder almost doubled the score three minutes later when his 25-yard drive flew wide of the post.
The Italian side enjoyed the majority of the play during the half and went close through Antonio Candreva, who headed wide after being found by Keita's cross.
Ludogorets had to wait until the 27th minute before creating their first opportunity of the match, when Virgil Misidjan crossed for Roman Bezjak, whose bicycle kick was easily saved by Federico Marchetti.
Two minutes before the break, Candreva drilled a free kick wide the far post after attempting to find the bottom corner.