Hi,
The answer to your question is covered in Section 2.5 of the Magic Tournament Rules, the section on End-of-Match Procedure:
In single-elimination rounds, matches may not end in a draw. If all players have equal game wins at the end of additional turns, the player with the highest life total wins the current game. In the event all players have equal life totals (or are between games and the game wins are tied), the game/match continues with an additional state-based action: if a player does not have the highest life total, they lose the game. Two-Headed Giant teams are treated as a single player for determining a game winner.
In other words: after 5 turns are up, if both players have the same number of game wins, we look at the player's life totals. If one player has a higher life total than the other, the player with the higher life total wins. If their life totals are tied, then you continue playing the game with the new state-based action of “If you have the lowest life total when state-based actions are checked, you lose the game”.
Nathan Long
Magic Rule NetRep