When is first player decided in 1v1 - before or after battlefields are revealed?

Ruling: First player is determined after battlefields are placed. Battlefields are placed face-up in the battlefield zone (a public information zone) before turn order is chosen.

Sequence:

• Legend revealed
• Chosen Champion revealed (already set during deck registration, modified during sideboarding)
• Battlefields chosen and placed face-up
• Decks shuffled and presented
• Turn order determined (first player chosen)
• Opening hands drawn
• Mulligans

Nuances:

• Many players incorrectly hide battlefields and reveal them after choosing turn order, but this is not supported by the rules
• Battlefields are public information once placed in the battlefield zone, so there is no rule requiring them to be "revealed" - they are simply placed face-up
• During sideboarding (between matches), you do not know your opponent's battlefield before making sideboard decisions