Heather Watson and Laura Robson have become the first two British women to be ranked in the top 50 in the world at the same time for more than 25 years.
Watson is 47th and Robson 50th in the Women's Tennis Association rankings.
Victoria Azarenka, from Belarus, is the world number one and will be the top seed for this month's Australian Open.
In the men's rankings, Andy Murray remains the world number three, behind number one Novak Djokovic, from Serbia, and Switzerland's Roger Federer.
Jo Durie and Sara Gomer were the last two British women to have been ranked in the top 50 at the same time, in 1987.
The latest rankings are career highs for Watson - who broke into the top 50 for the first time in 2012 - and 18-year-old Robson.
Watson, 20, is hoping to be fit for the Australian Open, which begins on 14 January, after withdrawing from warm-up event the Moorilla Hobart International with an elbow injury.
Robson has risen 81 places in just over a year after reaching the fourth round of the US Open in September.
Durie reached number five in the world and was a semi-finalist at the French and US Opens in 1983 while Gomer's highest ranking was 46.