mac book pro retina 2013 attempts to shutdown, then force shut down, unless Fn key is pressed continuously. no water damage, no liquid involved.

Hi, Today my macbook pro retina 15 nich early 2013 started a strange problem. After around 15 seconds of log in, the shutdown window pops up as if the power button was pressed. even before i can press cancel, it force shuts down as if the power button has been pressed for more than 4 seconds. I saw forums with similar problem in macbook air but it was all related to water spillage or similar issues. well in my case, there was no spillage, no water damage, no liquid near the macbook ever other than a damp apple cloth to clean the screen. when i press the fn key, it postpones this force shut down. it is still under warrenty, anyone knows how long does it take to repair? or is there any other solution possible?

I'm running os x 10.8.5