Andy Roddick admitted he was stunned by his rapid rise after he was confirmed as 2003 world number one.
Roddick is the sixth American to top the year-end rankings
The 21-year-old American is assured of finishing the year on top following Andre Agassi's win over number two Juan Carlos Ferrero at the Masters Cup.
Roddick becomes the sixth American and just the 13th player to finish the year at the top of the rankings.
"It's extremely surreal," said Roddick. "At the beginning of the year, I just wanted to make it here."
Ferrero's defeat in Houston saw the Spaniard make an early exit from the season-ending event for the top eight players in the world.
Roddick becomes the youngest year-end number one after Lleyton Hewitt, and joins fellow Americans Andre Agassi, Pete Sampras, Jim Courier, Jimmy Connors and John McEnroe.
"I'm incredibly honoured to finish the year as number one, but I'm probably more proud to be the sixth American ever," said Roddick.
"It's hard to reflect on what a year this has been but I'm incredibly fortunate to be playing at home in the United States.
"To be able to share all of this with my friends and family is better than I could have imagined."