Andre Agassi became the oldest player to top the world rankings after crushing Jurgen Melzer to reach the final of the US Clay Court Championships on Saturday.
The 32-year-old Agassi continued the form that has brought him 22 victories in 23 matches this year, overwhelming Austrian Melzer 6-4, 6-1 to set up a final clash with fellow American Andy Roddick.
"I don't think that I can quite find the words," Agassi said. "It's an amazing feeling because it's the result of a lot of hard work and a lot of determination, and a lot of decision making, a lot of success in many different arenas against many different opponents.
"It's a bit overwhelming to have it all come together at one moment."
Agassi, winner of eight Grand Slam titles, first topped the rankings in 1995.
He replaces Australian Lleyton Hewitt as the world's top-ranked player, a position he last held in September 2000.
"It starts to sink in earlier and earlier the older you get, you are sort of more aware of the fewer opportunities, you're surely more appreciative of the accomplishments," Agassi said.
Agassi, the 1992 Wimbledon champion, slumped to No 141 in the rankings in 1997 following a dramatic slump in form but recovered to climb to number six by the end of 1998.
He won the 1999 French and US Open titles, becoming only the fifth man in history to win the Grand Slam when he triumphed at Roland Garros, and has been Australian Open champion three times in the past four years.