The “problem kid” charged in a suspected knockout-game attack on a 62-year-old Bronx straphanger claimed the vicious assault was in self-defense, court papers show — as he was ordered held on $20,000 bail.

Deshaun Smith, 21, is charged with assault, harassment and reckless endangerment for allegedly punching the unsuspecting victim and knocking him onto the tracks at the 149th Street station Sunday night, according to police…

“Is it looking bad for me?” Smith allegedly asked cops of the case, according to the complaint. “My thing is, he said something to me inappropriate first. I was defending myself. Can I just say one thing before I get inside? I was defending myself…

The victim’s wife has told The Post that her husband was minding his own business on his way to work when he was struck in the unprovoked attack, which police sources said may have been part of the twisted online “knockout game” challenge targeting strangers.