Tennis legend Boris Becker has found a second career while locked away in an English slammer ... he's become a popular prison yoga instructor!
German tabloid Bild, citing a source close to the 54-year-old, reported this week that the convicted criminal has begun to thrive while behind bars ... teaching fitness routines to his fellow prisoners -- and losing a ton of weight.
The outlet reports Becker has made fast friends with inmates at Huntercombe prison in England, hosting yoga classes for around 45 people.