The previous time Baltacha was Britain's number one was in 2005
Elena Baltacha has overtaken Katie O'Brien as British number one.
The 26-year-old is at a career-high 85th in the latest WTA world rankings following a quarter-final appearance at a tournament in Japan last week.
O'Brien is now 89th in the rankings and Anne Keothavong, who is sidelined with a knee injury, is 99th.
"I value the British number one spot so much more this time because we're all much better than we were. There's a lot more value in it now," said Baltacha.
Baltacha has not been Britain's top-ranked player since 2005, when she reached the third round of the Australian Open.
Britain's women have enjoyed an encouraging run of success in 2009, with Baltacha, O'Brien and Keothavong all hitting career-high rankings inside the top 100.
Heather Watson followed in Laura Robson's footsteps as a junior Grand Slam winner at the US Open.
Baltacha's fellow Scot Andy Murray is British number one on the men's side.