* Second seed breezes into fourth round * Spaniard unbeaten on hard court in 2013 (Adds quotes, details) By Larry Fine NEW YORK, Aug 31 (Reuters) – Second-seeded Rafa Nadal has not lost serve once in three one-sided victories at the U.S. Open this week but feels his service game needs improving in his drive for another title at Flushing Meadows. Nadal may appear to be nitpicking given that his 6-4 6-3 6-3 battering of Ivan Dodig of Croatia on Saturday sent him into the fourth round and extended his 2013 hard court record to 18-0. …