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Yes, but you need to a have a North Amercia Hearthstone Account. Hearthstone has a few regional servers. Playing in your server provides better connection and game experience, but at the moment, a lot of players play across the world. The top players have accounts on NA/EU/Asia.
When players enter a match, the client allows them to play with up to 9 pre-made decks. Only these decks may be used in the match. However, if a player cancels the challenge, they then have the ability to alter his/her decks, which can provide an unfair advantage. Thus, sideboarding is not allowed in most tournament settings.