The first quote (paraphrase, actually) comes from his discussion with the Japanese leaders prior to the decision to go to war with America:
If you say it is to be war, I can promise you for a period of six months I will run wild in the Pacific, and will give you an uninterrupted string of victories. If the war lasts beyond that I have no hope of victory whatsoever".
The second quote occurred after news of the successful Pearl Harbor attack reached him. He reportedly recorded the words in his personal journal.
I fear all we have done is awaken a sleeping giant, and fill him with a terrible resolve.
I don't know if Yamamoto realized at this point the declaration of war was delivered after the attack or not, but both quotes taken in the context of history are chillingly accurate nonetheless.