Hey,

I googled and didnt find anything about this subject but in the achievements list i see you can unlock these two players.
Yesterday I was doubling and got really pwned by this duke. He was a goddamn volley master.

If anyone knows how to unlock'em please share it with us.

Thank You

Allright I have found a way to unlock'em. Finish tournament mode without losing a game(always 2-0). Then you can play against one of them and if you win you unlock that player.
Difficulty level:
Very easy = noone I think
Easy = Duke lvl 0 (silver medal)
Normal = King lvl 0 (silver)
Hard = Duke lvl 1 (gold)*
Very Hard = King lvl 1 (gold)*

*not sure since im stuck at the final stage of the hard difficulty level.

Greetings,

Zeppos

And another reply to myself:
Forget the things I wrote above. Win the tournament without losing games(dont retry) and you get a bonus round with duke/king. It seems random which opponent you get. easy is lvl 0 , normal lvl1, hard lvl2 and I suppose veryhard is lvl 3. Dont really know if you beat them your unlock willl be of the same level.
Greetz to myself