Di Grassi, who retired from Saturday's race with a slow puncture after contact with Oliver Rowland, swept to the top with a 1m06.133s lap, logging the best first and third sectors of the morning as fine margins split the field - the top 21 all within 0.8s of di Grassi's time.
He was followed into second by Mortara, who had headed a tightly contested FP1, as the Swiss driver continued his excellent Berlin form and eclipsed Rowland - who had led the opening proceedings of the session.
