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Dereck Chisora has always had good stamina for a heavyweight. Going back to 2010 and his best win to that date, he outlasted Sam Sexton to stop him in nine having set a good pace. For that fight Chisora weighed in at 244 pounds.
Then against Tyson Fury, Chisora weighed in at a generously proportioned 261 pounds, which clearly slowed him down and worsened his stamina. In his most recent defeat to David Haye in 2010, Chisora was 247 pounds and still looked like he was carrying some excess baggage.
There was a real change in the last two fights before Johnson, Chisora weighing in around 235 against Ondrej Pala and Edmund Gerber. In both those fights Chisora was unrelentingly active, and that took Pala and Gerber far out of their comfort zones: Gerber getting stopped in five, Pala in three.
It looked like Chisora had hit on a new strategy based on being leaner and meaner—able to keep up such intense pressure on opponents that they would simply wilt.
Kevin Johnson had never been stopped in four defeats and was to some extent the perfect test for Chisora's game plan. It looked to be working, too, when Chisora decked the American in Round 5.
But after that Chisora wasn't able to continue the pressure for 12 rounds and eventually looked exhausted in the last three, even allowing the low-output Johnson to nick a round or two late on.
Sadly for the Finchley fighter his better physical condition has not translated into a significant improvement in stamina down the stretch and he will not be able to rely on his good stamina to win fights at the highest level. Indeed, against top opposition, he may even need to consciously pace himself better.