Hamilton leads the race closely followed by team mate Fernando Alonso and Ferrari's Felipe Massa.
Leading from pole, Hamilton, whose only experience with the Brickyard before Friday was through video games, posted a succession of fastest laps to open up a four second lead on Alonso.
But after the first pit stops the Spaniard began to give chase, slicing the gap and filling the Briton's rear view mirrors.
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Formula One 2007: Complete coverage