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YG's Friday is off to a troubling start -- cops showed up on his doorstep before sunrise armed with a search warrant ... TMZ has learned.

Law enforcement sources tell us L.A. County Sheriff's deputies arrived at the rapper's San Fernando Valley home around 4 AM, and they're still there now. YG is home, and we're told they have a search warrant and an arrest warrant related to a robbery case.

Of course, YG's SUV was recently connected to a murder case. We broke the story ... the L.A. County Sheriff's Dept. says it's close to cracking the case, but has asked the public for an assist. As we told you, the rapper's Escalade was involved in a high-speed chase back in July through L.A. streets -- and it ended with someone in the vehicle shooting and killing a 65-year-old man on a bicycle.

YG told us at the time ... "I was nowhere near the scene of this incident and had been in a recording studio all day in Hollywood. I was there until after midnight on the Fourth of July and didn't learn of these events until after they happened."

It's unclear if the Friday morning raid is related to that case.

A couple weeks after the shooting, a Hollywood Hills home leased by YG was raided and at least one person was arrested for a weapons charge. The rapper was not present at the house during that raid.