50 Cent's 'Final Lap' tour just summoned its biggest guest yet -- Eminem, who gave the crowd the surprise of a lifetime just outside his hometown.

The tour stopped in Clarkston, MI Sunday night at Pine Knob Amphitheater and you can hear the place practically erupt when Em hit the stage to perform his "Patiently Waiting" verse from Fif's classic LP "Get Rich or Die Tryin."

Em also stuck around to perform "Crack a Bottle" from his own "Relapse" album, and fired up the crowd in celebration of the ongoing HipHop50 celebrations -- a milestone heavily influenced by both artists.

As you've surely heard hundreds of times ... Dr. Dre signed Em, who signed 50 and together they all sold like a kajillion records together!!!

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