Wimbledon champion Linda Noskova upset Coco Gauff to storm into the last four of the WTA Finals at Indian Wells on Tuesday as Jessica Pegula reignited her challenge with victory over Ukraine's Marta Kostyuk.
Czech eighth seed Noskova took advantage of a shaky serving performance by Gauff to complete a 6-3, 6-3 victory in group one to book her place in the semifinals.
Noskova broke Gauff's serve four times on her way to claiming the first set, and then grabbed the decisive break early in the second before sealing victory with her ninth ace of the match.
"I'm very happy, obviously every win counts in this competition," Noskova said afterwards. "Coco is a big competitor so I had to be ready for everything that would come my way today.
"You have to play to the very last point with Coco because she can always help herself out with her serve and fighting spirit. I felt like I played great to put some pressure on her," added Noskova, who defeated Ukraine's Kostyuk in her opening group match on Sunday.
The defeat leaves Gauff and Pegula vying for the last remaining semifinal berth from group one.
Pegula, beaten by Gauff in her opener Sunday, overcame a topsy-turvy second set to defeat Kostyuk 6-2, 7-5. Kostyuk is eliminated following Tuesday's defeat.
"I feel like I'm playing well," Pegula said. "I felt my match against Coco was a super high-level match and that was a great match today.
"It's so tricky with her because you just don't know what's gonna come -– she will just step in and inject pace, she'll come and kamikaze the net, she goes for returns or her serve can get hot. That's what makes it difficult to play her."
The 32-year-old American had looked in complete control in the first set, breaking Kostyuk in the first and seventh games to take the opener 6-2.
A chaotic second set began with four consecutive service breaks before Kostyuk held for a 3-2 lead.
The Ukrainian then had a break point in the next game but was unable to take advantage as Pegula held for 3-3.
The next four games went with serve but Pegula finally grabbed the decisive break in the 11th game. She converted her fourth break point to take a 6-5 lead before holding for the match, sealing victory when Kostyuk smacked a forehand long.