Kostya Tszyu carried a painful rib injury in the weeks leading up to his IBF super lightweight world title victory against Sharmba Mitchell, The Daily Telegraph reported.

Tszyu, who stunned the boxing fraternity with a third round knockout of Mitchell in Phoenix on Sunday, had not had a bout for 22 months before the win.

Compounding an injury-laden two-year period was the fact Tszyu had badly injured his ribs in a sparring session three weeks before the fight.

Tszyu's trainer Johnny Lewis said the 35-year-old kept the injury quiet but it was initially feared he'd broken them and, after two postponements of a re-match with Mitchell, his camp felt his career would be over.

"The x-rays showed the ribs weren't broken," Lewis said.

"But they were very badly damaged.

Lewis said doctors in Australia and the US indicated while the injury was painful it was unlikely to get any worse and Tszyu was told he'd be OK to fight if he could live with the pain.

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