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Underdog Brook recovered from a slow start to outlast Porter down the stretch, with two judges awarding the bout to Brook 117-111 and 116-112, with the third official calling it 114-114.
Porter was relentless throughout the California encounter, frequently swinging with both hands while attempting to outmuscle Brook, who was competing in his first world-title contest.
However, despite seeming to dominate the opening exchanges, where he also opened up a cut above Brook's eye, he visibly tired during the middle part of the fight as Brook began to grow in confidence.
The Englishman caught Porter with a solid right hand at the end of the seventh, and that had a lasting effect on the American who was struggling to keep up his previous intensity.
The latter rounds were scrappy, with Porter the constant aggressor, but Brook was picking him off at range and that proved crucial with the judges.
The final scores may have been generous to Brook, but in overcoming the previously unbeaten American, the 28-year-old has opened up plenty of doors as he looks for lucrative encounters in the future.