Shields conqueror turns pro
Marshall outpointed Shields en route to becoming the world amateur champion in 2012.
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The 25-year-old dropped out of the Great Britain amateur squad in March after failing in her second attempt to win an Olympic medal in Rio last year.
Marshall is now set to base herself in Las Vegas to work with undefeated former world champion Mayweather's promotions company.
A potentially lucrative re-match with Shields, who won her first two professional fights and made history by becoming the first female fighter to top a premium television boxing card in the United States, now looms.
But Shields did not exactly welcome her old rival into the professional ranks, tweeting: "The best news I've heard in a while! Her ass is kicked! @MayweatherPromo set it up!"
She added in a later tweet: "Mannnnnnn he (Mayweather) just added fuel to the fire man! I'm going to light her ass up! Right in front of him!"
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