Berdych improved his return of serve A LOT - but he is still weak with volleys....
Thats why people call him choker. He is not choker , simply volleys are worst part of his game.
Because he can make volleys, even difficult ones, not under pressure. I've seen it, especially in davis cup. He isn't the best, but he would be able to win every doubles rubber with Steps last year if he was that bad at volleying. Just at pressure moments, he misses routine, easy, high volleys in singles.