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2. Ray Mercer
Adrenaline MMA 3
June 13, 2009 | Birmingham, Ala.
If James Toney is the most credentialed boxer to ever step into a cage, then Mercer -- the gold medalist at the 1988 Olympics and former heavyweight title challenger -- is not too far behind him; Ricardo Mayorga would also deserve a mention. Mercer knocked out future heavyweight champion Tommy Morrison and got in the ring with fighters like Larry Holmes, Evander Holyfield, Lennox Lewis and Wladimir Klitschko during a professional career that spanned nearly 20 years.

A prolific fighter, Mercer also had two appearances as a professional kickboxer in the K-1 organization, both of them losses. He fought backyard brawling legend Kevin Ferguson, aka “Kimbo Slice,” under MMA rules in 2007, but the fight was unsanctioned and thus does not disqualify him from inclusion on this list.

The MMA fight for which Mercer is known -- his one and only official bout -- came against former UFC heavyweight champion Tim Sylvia in 2009. Sylvia was on a rough run at the time, having been submitted consecutively by Fedor Emelianenko and Antonio Rodrigo Nogueira. He had losses in three of his last four fights. Still, Sylvia was a drastically more experienced mixed martial artist, and Mercer was an utter novice at anything other than boxing.

Mercer needed nine seconds and one thunderous right hand to put Sylvia to sleep. Despite the former UFC champion’s horrendous physical condition and seeming apathy, it was still an incredible upset.

Number 1 » He was the first American to win Olympic gold in a full international field -- two won in 1984 in Los Angeles, where the Soviet Union held out its athletes -- at the 2000 Olympics in Sydney, and he later took a bronze medal in Athens in 2004, along with a world championship in 2001.