* U.S. Open champion given workout on return
* Murray shows glimpses of New York form in second set (Adds quotes and details)
TOKYO, Oct 2 (Reuters) - Defending champion Andy Murray blew away the cobwebs with a 7-6 6-4 win over Croatian Ivo Karlovic on Tuesday in his first tennis match since winning last month's U.S. Open.
The Olympic gold medallist said winning his first grand-slam title in New York had forced him to change his objectives and become more "responsible."
"I was so, I don't want to say 'obsessed' with winning a grand slam, but it was so important to me," world number three Murray told reporters.
"Sometimes during the year in certain matches I haven't done myself justice. I felt after the U.S. Open (I needed to) reset some goals.
"I was always focusing on the next grand slam rather than every tournament and every match I played.
"Now I feel like I can hopefully concentrate better, take more responsibility for my performances in every tournament throughout the year."
The Scot said he felt no different having finally won his first grand slam after losing in four finals.
"It didn't feel a whole lot different (stepping on court as a grand slam winner)," said the 25-year-old.
"Hopefully with time that will come," added Murray, who reached his first Wimbledon final this year before exacting his revenge on Roger Federer in the Olympic final in London.
"I think where I will feel it more will be if I'm getting to the latter stages of grand slams. Hopefully I will start feeling more confidence to go on to win."
Murray had been scheduled to open his Japan Open defence against Gael Monfils but the Frenchman pulled out with a knee injury, to be replaced by lucky loser Karlovic.
He got a workout from the 2.08-metre Karlovic and was forced into a tiebreak in the first set which the Scot took 9-7 with a superb running forehand pass down the line.
After a quick wardrobe change, Murray, now in all black, patiently waited for his opportunity as Karlovic, beaten in qualifying by Swiss Marco Chiudinelli, blasted 18 aces.
Karlovic, facing three match points at 5-4 down, saved two with unplayable serves but Murray pounced on the third, closing out proceedings with a vicious backhand to the Croatian's body.
"It's always hard the first match after a few weeks off," said Murray, who had won all his four previous meetings against Karlovic. "I played well when I needed to."
Murray, who was on court for an hour and 38 minutes, will face Slovakia's Lukas Lacko in the second round.
Japan's Kei Nishikori survived a fright against countryman Go Soeda, the eighth seed squeaking through 4-6 6-2 6-3 to the relief of Tokyo organisers in two hours 29 minutes.
Nishikori, who reached his first semi-final of the year in Kuala Lumpur last week, next plays Spaniard Tommy Robredo, a 6-2 6-4 winner over Finland's Jarkko Nieminen.
Big-serving Milos Raonic hit 10 aces in a 6-4 6-4 victory over Czech Radek Stepanek, the Canadian's 40th win of the year taking him through to face Serbia's Viktor Troicki.
Russian qualifier Dmitry Tursunov piled more misery on Australian hope Bernard Tomic, recently given a dressing down by Davis Cup captain Pat Rafter, beating the 19-year-old 6-4 7-5.