Miami - Mardy Fish replaced Andy Roddick as the top-ranked American after beating Spain's David Ferrer 7-5 6-2 on Wednesday to move into the semifinals of the Sony Ericsson Open.
Fish, currently ranked 15th, will formally overtake his friend and world number eight Roddick when the ATP world rankings are updated next week.
Defending champion Roddick, struggling with a bronchial infection, made a shock early exit in his first game at Miami, losing to Uruguayan Pablo Cuevas on Saturday.
"I don't think I will ever feel I am the top-ranked American, Andy has had a pretty good career and he has always been top dog in my generation," Fish said of his former high school team mate.
"Top ten is something that I am aiming for and I will be working my butt off to get there."
Fish had a surprisingly easy passage to the last four, with the normally solid Ferrer fading fast in the second set in the Florida heat.
The Spaniard lost the first set after double faulting on break point to allow Fish to go ahead 6-5.
Fish will face the winner of Wednesday's other quarter-final between Novak Djokovic, unbeaten in 21 straight games this year, and South African Kevin Anderson.