I've said it in the other thread and here's how I see it.
Return is important but it alone won't get you far. Most of the French players can break him (Santoro, Clement, Llodra, Simon, Monfils, even Benneteau come to mind) cause they are pretty much all good returners can put a lot of his serves into play.
But equally important is to be safe on your own service games and to try only to maneuver him around the court, not go for winners and risk to much. You also have to deal well with slice, pretty much most of the "smaller" (shorter) guys do as they are lower to the ground.
arlo has a 20-23 score against Frenchies but 38-22 against the Yanks- is there something to it? It think so. French guys can return serve and aren't all ballbashing behind their own
Blake, for example has an excellent return but it never gets him anywhere against Karlo cause he always gives chances when serving and is poor when it comes to playing big points.
The Spaniards, who aren't particularly known for liking to play against big serving guys also have a positive record against Karlo (22-21), I guess cause they play a safe game, they don't give away cheap points,..at least that's how I figure it
Bad matchups (except the French
)- Kiefer, Melzer, Murray, Nalbandian, Stepanek, Stakhovsky, Stadler
) come to mind