Defending champion Andy Murray was quickly back in the groove in the Japan Open in Tokyo yesterday, beating Croatian Ivo Karlovic in his first match since winning last month's US Open in New York.

The Olympic gold medallist, who triumphed 7-6, 6-4, said winning his first Grand Slam 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," said Murray, who now faces Slovakia's Lukas Lacko in the second round.

"During the year in some matches I haven't done myself justice. I felt after the US Open I needed to reset some goals.

"I was always focusing on the next Grand Slam rather than every match I played.

"Now I feel I can hopefully concentrate better, take more responsibility for my performances in tournaments."

The Scot said he felt no different having won his first Grand Slam after having lost 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 Slams. Hopefully I will start feeling more confidence to win."

Murray had been scheduled to open his defence against Gael Monfils, but he pulled out with an injury, replaced by lucky loser Karlovic.

"It's always hard the first match after a few weeks off," said Murray, who won his four previous meetings against Karlovic.