"Thinking is everything in this game. It is hard to express what it is like, but there have been sleepless nights," Tendulkar said in an interview with this month's issue of Cricinfo magazine.

"There have been days full of frustration where you just want to get back in action but the body does not cooperate even if your mind is ready to go out there and do it," said Tendulkar, who underwent shoulder surgery in March.

He described his latest battle with fitness as the "greatest test of character" but added he was determined to win.

"If you are unlucky, you will get injured even if you are the fittest guy in the world. What I don't want to lose is the desire to get back in action and the hunger to go out there and perform," he said.

"When you are constantly attacked by injuries, it is not easy to start thinking like a 24-year-old with a fit body. It's got a lot to do with rhythm as well."

Tendulkar said he is fortunate to have a coach like Greg Chappell who understands.

"Greg Chappell was a top cricketer, one of the best to have played this game. It is important to have someone around who has played a lot of cricket at the highest level," he said.

"Technically, to a certain extent, one can progress, but mentally you can get better each time you go out into the middle. That is where Greg chips in."

Tendulkar is the leading run-getter in the shorter version of the game with 14,146 in 362 one-dayers and 39 hundreds.