The 28-year-old, who has already defended the belt twice since winning it from Ireland's Andy Lee in 2015, was a convincing victor on all the judges' scorecards - 120-108, 117-111, 118-110 - and remains undefeated.
In what HBO's Max Kellerman correctly labelled a "masterclass in boxing", Saunders immediately negated the Canadian's significant power with supreme footwork, befuddling the former champion with swift counter-left hands over the top and sharp right hooks around the guard.
Lemieux, meanwhile, claimed to have injured his left hand in the second round, but lacked humility - and indeed a grasp on reality - when he refused to credit Saunders for his victory. "I like to come into someone's backyard because I perform better".
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The seventh round showcased Saunders' dominance, the Brit landing numerous blows to a bleeding Lemieux. The Canadian Press had it 118-110.
The defeat is the fourth of Lemieux's career, who had won each of his four fights since being stopped by unified middleweight champion Gennady Golovkin in 2015. Only the third and sixth rounds were close until the final two rounds that Saunders mainly spent dancing out of Lemieux's reach. "It's alright performing a million dollars but they won't want to fight me fighting like that", Saunders told BT Sport.
The first six rounds were all taken comfortably by the former British Olympian, who for 18 minutes danced around a large ring while peppering his plodding opponent with twos and threes, taunting the Montreal native for his frequently crude lunges and static head movement.
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"It feels very good", said Clayton.
Clayton took control early and was never in trouble against the Argentine, who kept his record of never being stopped before the limit intact even though he had to take a knee after a right to the ribs in the ninth and was dropped for an eight-count by a right uppercut in the 10th. "I knew he was tough". Fight me now. Now fight me, cos we'll be seeing a different story. His goal is to get into position for a world title bout by the end 2018 or early in 2019.
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