Khazri converted from
But Sarr ensured Rennes avoided a fifth Ligue 1 defeat this season with a fine finish in the 32nd minute. Although had Hatem Ben Arfa converted a 66th-minute penalty, they would have bagged their fourth victory in the league.
The effort increased the Senegal youngster's tally to three goals in seven league matches this term while his Tunisian counterpart notched his fifth goal in nine league outings.
Rennes remain unmoved from the 10th spot with 12 points from 10 games while Saint-Etienne dropped to sixth in the Ligue 1 log with 16 points.