Poland's Iga Swiatek will succeed Ash Barty as world No 1 following a 6-2 6-0 win over Viktorija Golubic in the second round of the Miami Open.
The 20-year-old, who becomes the first Polish player to achieve the feat, was only elevated to world number two after winning the BNP Paribas Open at Indian Wells last week, and will take top spot following Barty's shock retirement on Wednesday.
She will officially become the youngest player to make her No 1 debut since Caroline Wozniacki, who was 216 days younger, in 2010, when the new rankings are published on April 4.
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Swiatek is on a career-best 12-match winning streak and has a tour-high 21 wins this year, with WTA 1000 titles in Doha and Indian Wells.
"I'm pretty sure that tomorrow the sun will shine exactly the same way as today but for now...it feels surreal. No.1!!" Swiatek wrote on Twitter following her win.
Her No 1 ranking will be recorded officially once the tournament is over and Barty is removed.
Swiatek told a press conference: "The thing is, I never really imagined that moment because, truth to be told, I was working day-by-day and playing tennis well. But I never had the strong belief that it can actually happen. So, it's
even more surreal for me.
"I loved every moment of it. It doesn't matter for me if there were fireworks. The crowd was really supportive, they were really excited. I'm just soaking everything in because I didn't have any expectations."
The Pole is the 28th woman to hold the No 1 ranking since the computerised system was introduced in November 1975.