Andy Roddick will lose his world number one ranking following his Australian Open quarter-final defeat to rejuvinated Russian Marat Safin.
The American's epic five-set loss means that either Roger Federer or Juan Carlos Ferrero will go top, depending on their other results in Melbourne.
Federer takes on David Nalbandian and Ferrero faces Hicham Arazi when men's action resumes on Wednesday.
"One's a nice number to have but I have time to get it back," said Roddick.
He added: "No-one can take away from the fact that I was there and that I did have it.
"I've got a feeling it's going to be jumping around a bit this year, but that's what makes it exciting."
Roddick became the youngest American to finish the season as world number one when he claimed top spot in 2003 at the age of 21 years and two months.