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Not including anything that is the result of using a tennis racket (otherwise any glowing shot guy is automatically impossibly physically strong), who would be the physically strongest guys in Prince of Tennis? Here would be my picks:
Oni - Can lift a boulder.
Sanada and Kawamura - Can catch a guy falling with one hand.
Kintaro - Can lift a motorcycle.
Duke - He's probably very strong but he's done nothing that isn't related to tennis and is not eligible.
Akutsu - He caught Kawamura while airborne and ripped a hole in the fence, but I'm not familiar with how that compares with the above feats of strength.
So looking at the list, it seems like there aren't too many feats of strength that doesn't involve a tennis racket.