Andy Roddick became the new world number one with victory over Tommy Robredo at the Paris Masters.
The 21-year-old will leapfrog Juan Carlos Ferrero when the rankings come out on Monday, and will also overtake the Spaniard in the Champions Race.
Ferrero was earlier knocked out of the Paris event by Jiri Novak but he and Roddick will go head-to-head for the year-end top spot at the Masters Cup.
"Every match win or lose could be the difference," Roddick warned.
"But I've got there and no one can take that away from me - ever.
"It never crossed my mind when I was a kid that I might be number one. It's definitely something I never thought would happen."
US Open champion Roddick, the fourth youngest world number one in history, can increase his lead in both the rankings and the Champions Race by progressing further in the tournament.
But Ferrero said he was ready to fight to reclaim the top spot.
"I will do everything in my power to be world number one," said Ferrero.
"Andy has to play well at the Masters Cup to be world number one and we will fight for it.
"It has always been a goal of mine to finish the year as number one."
Roddick, the 22nd player to hold the world number one spot since rankings began, saw off Robredo 6-3 6-4 to set up a quarter-final against Jonas Bjorkman.
Meanwhile, an emotional Novak described his win over an error-strewn Ferrero as "one of the greatest of my career".
"I was playing great tennis today and I had nothing to lose," he said. "I was playing the number one player in the world and beating him doesn't happen every week."
Martin Verkerk almost caused another huge upset, but could not take any of the four match points he earned against a surprisingly nervy Roger Federer.
Federer advanced with a hard-fought 6-7 (3-7) 7-6 (14-12) 7-6 (8-6) victory.
Earlier, Paradorn Srichaphan's slim hopes of reaching the end-of-season Masters Cup vanished when he lost to Hicham Arazi.
Srichaphan needed to win the tournament to dislodge David Nalbandian from the eighth and final qualifying place, but stumbled to a 7-6 (7-3) 6-3 defeat.
Germany's Rainer Schuettler battled to a 6-4 7-5 victory over Nicolas Massu.
The fifth seed gained revenge for the defeat he suffered at the hands of Massu in their only previous meeting.
Schuettler will meet Andrei Pavel in the last eight after the Romanian's opponent Guillermo Coria withdrew from the tournament with a fever.
Later on Thursday, British number one Tim Henman plays Gustavo Kuerten for a place in the quarter-finals of a Masters Series event for the first time this year.