* Nadal still yet to drop a set in the tournament * Azarenka overcomes tough challege from Cornet * Kvitova and Kuznetsova both beaten (Updates with Nadal winning) By Julian Linden NEW YORK, Aug 31 (Reuters) – Rafa Nadal, growing in confidence with each match, blasted his way into the fourth round of the U.S. Open on Saturday with an ominous display. The Spaniard turned on a masterclass of power and precision at the last grand slam of the year as he brushed aside Croatia’s Ivan Dodig 6-4 6-3 6-3. …