* 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.
But the 27-year-old Spaniard knows what it takes to win grand slams and the U.S. Open in particular.
"I didn't feel that I was serving great," said 12-times major winner Nadal, the 2010 U.S. Open champion. "I served OK, not bad.
"In important moments I did a few good serves, but I am winning because I am playing well from the baseline and I am making the right decisions in the right moments."
Nadal, the undisputed King of Clay after winning his eighth French Open earlier this year on the red clay of Roland Garros, continued to show his hard court prowess as he ran the dogged Dodig relentlessly on Arthur Ashe Stadium.
"I think I played a correct match today. Happy 6-4 6-3 6-3 against a player who came to this (stage of the) tournament winning against tough opponents like (Nikolay) Davydenko and (Fernando) Verdasco.
"To be able to win against him in straight sets is great news. I played better today than in the previous matches. Always a positive thing."
Nadal smacked 37 winners in the two-hour eight-minute victory, nearly double the number registered by Dodig, and showed his versatility by winning 14-of-17 forays to the net set up by his ferocious groundstrokes.
Nadal has yet to have his service broken, but he feels that statistic might be deceiving. "Will happen," he said, drawing a laugh from reporters.
He completed a career grand slam by winning the 2010 championship armed with a booming serve that hit speeds over 130 miles an hour (209 kph) after making a subtle grip change.
The Mallorcan is having a remarkable comeback year after being idled for seven months due to a knee injury, but the power has not been turned on for his first deliveries this week.
While he faced only two break points against Dodig, Nadal had his fastest serve clocked at 123 mph (198 kph), with his average first serve speed 108 (174 kph).
"Is working, but I am not serving at 130 miles every time," said Nadal, who instead is relying on placement and consistency to gain an upper hand on his service points.
"It's working because I am playing well from the baseline. It's working because I am serving with the right percentage. It's working for the first three matches. We'll see if I am able to keep working."
Nadal's formula has worked in 2013 as he has posted a 56-3 match record including a run to the French Open crown.
The world number two also relies on a prodigious work ethic.
"It's something I am going to work a little bit more tomorrow," he said. "I need to serve a little bit better, and I hope I gonna do good."
Nadal will play 22nd seed Philipp Kohlschreiber of Germany for a berth in the quarter-finals. The German defeated 13th-seeded American John Isner 6-4 3-6 7-5 7-6(5) to advance.
(Reporting by Larry Fine, Editing by Gene Cherry)