Rob Kardashian -- No Time for 'Dancing With the Stars' Rehearsals

Rob Kardashian has a nasty case of pre-dance jitters -- and it's starting to piss off his "Dancing with the Stars" partner.

Sources close to the show tell us Rob is very nervous and has refused to rehearse longer than 2 hours with Cheryl Burke -- while most of the contestants are logging more than 5 hour practices.

We're told Cheryl is "fed up" with what she perceives to be Rob's lack of interest in the show.

But Rob is still talking a big game ... because we're told he's bragging constantly to big sis Kim that he will top her 'DWTS' performance by making it past week 3.

Note to Rob: You don't have Kim's "assets." Get to practice.