Serena Williams became the oldest woman to claim the top ranking when she won her quarter-final match at the Qatar Open.
The 31-year-old defeated Petra Kvitova 3-6 6-3 7-5 to ensure she will become world No 1 again on Monday regardless of what happens in the latter stages of the tournament.
She was 4-1 down in the deciding set against the Czech but won six of the last seven games, meaning she will surpass 30-year-old Chris Evert's record.
Williams, who returns to the top spot for the first time in two years, will take over from Australian Open champion Victoria Azarenka.
"I'm so sensitive these days, I'm always crying, but I never thought I would be here again," she said.
"When I was down I heard people cheering for me and I don't get that all the time. I never thought I would be number one again."
She will face Maria Sharapova in the last four after the Russian brushed aside Australia's Samantha Stosur 6-2 6-4.
Going into the match Sharapova held an impressive 11-2 record against Stosur and was never threatened during a routine 82-minute victory.
Azarenka saw off Italian Sara Errani 6-2 6-2, matching Sharapova's feat of not dropping a set so far this week, and will take on fourth seed Agnieszka Radwanska, who defeated Caroline Wozniacki 6-2 7-5.