Depending on Sypherz decision, either he would win the game, or I would win the game. I had a monster with 2300 attack on the field but also a field card granting 300 attack. Sypherz only had 2400 lifepoints and I knew he had Magic Cylinder face down on the field. I attacked him with the monster and he accepted the hit, lowering his lifepoints by 2300. I reminded him of the field card lowering it by another 300 (due to not countering) and losing the game.
It's games like those that make these so exciting. I had offered him a rematch for one game due to a ruling I found on a card I played in the 2nd turn of our 2nd match, which I found questionable and sought confirmation/explanation.
Cosrob is correct.
However, say you have zero cards left, and your opponent plays one day of peace. (Where you are forced to draw one card), you also deck out.