Swisscom Challenge, Zurich
Justine Henin-Hardenne replaced Belgian compatriot Kim Clijsters as world number one with victory in the final.
The 21-year-old crushed Jelena Dokic 6-0 6-4 to win her eighth title of the year.
Henin-Hardenne is the 13th woman since computer rankings were introduced in 1975 to reach the number one spot.
"It's a great day for sure. I always gave everything I have to be number one. It's an amazing feeling," she said.
"I started tennis when I was five and to be number one was a dream."
However, Henin-Hardenne's reign will only last one week as she has elected not to defend her title in Linz next week, losing her valuable points.
She missed out on taking the prized ranking last week when she lost to Clijsters in the Filderstadt final, but she was ruthless against Dokic.
The first set was over within 15 minutes after Henin-Hardenne shattered Dokic's confidence, allowing her to collect just four points.
Dokic, who defeated Kim Clijsters in the semi-finals, put up more of a fight in the second but Henin-Hardenne secured the crucial break in the fifth game and won the match in 50 minutes.
Dokic said she was pleased to have shown something like her best form in the tournament, after a desperately disappointing year.
"I didn't play as well as yesterday (against Clijsters)," said Dokic.
"She served better than me, she was more solid than me, but for me to come as far as I have, it's a great tournament.
NUMBER ONES SINCE 1975
A Sanchez Vicario
"I surprised myself, not just by winning but the way I played."
Henin-Hardenne confirmed she will not defend her title in Linz nex week as she looks to get some rest, knowing that will allow Clijsters to regain the number one ranking.
"I will keep it for a week now then we will see," she said.
"I decided not to go to Linz and I am sure it was the right decision because I have to be 100% in a few weeks for the WTA Championships (in Los Angeles in November).
"Becoming number one is because you won Grand Slams and you won big events. It is not a result.
"It is very important, but what helped me to be number one was winning the French Open and the US Open and all the other tournaments.
"So the (WTA) championships are my next goal. I will take care of my body in the next two weeks and then maybe I can finish the year as number one.
"Being at the top is not easy but staying at the top is going to be the most difficult."