The defeat leaves Gauff and Pegula vying for the last remaining semi-final berth from group one.
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 semi-finals.
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 semi-final berth from group one.
