Iga Swiatek's hopes of finishing as the year-end world number one were dashed after she fell to defeat against Coco Gauff at the WTA Finals on Tuesday. (More Tennis News)
Gauff earned her first triumph over Swiatek since the 2023 Cincinnati Open, emerging a 6-3 6-4 victor to reach the final four of the competition in Riyadh.
It means Aryna Sabalenka is now guaranteed to top the WTA rankings at the end of a calendar year for the first time in her career.
World number three Gauff finished the encounter with 10 winners to 33 unforced errors, 11 of which were double faults. Swiatek outpaced Gauff with 15 winners, but struck 47 unforced errors.
At the age of 20 years and 237 days old, Gauff is the youngest American to reach back-to-back semi-finals at the WTA Finals since Tracy Austin (1979 and 1980).
She is also the first player under the age of 21 to win both her first two round-robin matches at the year-end competition since Ana Ivanovic and Maria Sharapova in 2007.
"It feels great and despite our head-to-head I was still confident and I knew if I could find my game, I knew I had the chance to close out the match," Gauff told Sky Sports Tennis.
"I was just trying to be resilient and play it deep. The conditions are tough and the altitude is tough.
"I was trying not to give her too many unforced errors. It's just a game of cat and mouse in these conditions."
In Tuesday's other match, Barbora Krejcikova ended Jessica Pegula's hopes of qualifying for the semi-finals of the tournament with a straight-sets triumph.
The Wimbledon champion needed just 69 minutes to secure a 6-3 6-3 victory, serving 11 aces throughout the contest compared to Pegula's one.
World number 13 Krejcikova became the lowest-ranked player to win a match at the WTA Finals since Magdalena Maleeva (ranked 17th) defeated Daniela Hantuchova in 2002.
The Czech will also have confidence of claiming the crown in Riyadh having won her previous events in Tallinn, Ostrava, Dubai and Wimbledon when beating an opponent in the WTA's top 10.
"I was fighting for every ball and I felt that I really have to play my best tennis," said Krejcikova, who picked up just her 20th match win of the season.
"I was trying to be really solid and trying to put as many balls to the other side as I could.
"I’m definitely proud. I had some very high parts of the season, especially winning Wimbledon. That’s something indescribable.
"And being here in the Finals is a huge privilege. It’s nice to get the win and still be part of the event."
