LAS VEGAS—Brock Lesnar stopped heavyweight champion Randy Couture in the second round of their fight at UFC 91 on Saturday night, earning a technical knockout when the referee stepped in as he kept punching after Couture went to the ground.

Lesnar, a former NCAA wrestling champion and WWE star, overwhelmed the 45-year-old Couture in just his fourth pro fight. Lesnar sent him to the canvas in the second round with a glancing blow that landed behind the ear and wasted no time in landing numerous hammer fists. Couture was never able to recover and the fight was stopped at...

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