The Hebrew word "shabath" is translated into English as "rested". ("shabath" is where the word "Sabbath" comes from).
"Shabath" DOES NOT have any meaning of being tired. The more correct meaning is "desist" or "to cease ones labors". It means the work is done because there is no more to do.. It does NOT mean that God was tired, it means that He had completed His tasks.
God's sense of time is NOT like our sense of time, God can see the past, the present, and the future as if they were all happening at some generic point in time called "now". HOW He does that we don't know, because our minds are too tiny to conceive of that happening.
But it is YOUR limited thinking that causes you to ask this question. YOU are particularly unable to conceive of such Godly matters, and thus you draw wrong conclusions because you try to limit God's capabilites into things YOU understand, instead of making the effort to understand the things of God that are MUCH greater than you..
Of course, we KNOW that some other time definition than "earth day" MUST have been used, as God didn't make the Sun until the FOURTH "day". Until then, God Himself provided the light necessary for the plants to grow, as they were made on the THIRD day!
The point is, God is TRANSCENDENT to the Universe. He is IN and AROUND the Universe, but He is not PART OF the universe. He doesn't live on some planet (like the Mormon's think), as that would limit God to the physical scale of the planet itself..
God, being transcendent, can define "a day" as ANYTHING He wants to!