Mirra Andreeva won 11 games in a row as she thrashed world number two Aryna Sabalenka to reach the last four at the WTA Finals.
French Open champion Andreeva was 4-1 down in the opening set before producing some superb tennis and taking advantage of an increasingly erratic Sabalenka performance.
The 19-year-old Russian went on to deliver a statement 6-4 6-0 victory over four-time major singles winner Sabalenka.
As poor as Sabalenka was, Andreeva produced a mature, intelligent performance to shut her out of the match.
She used loopy high balls to initially wrong-foot Sabalenka and then showcased her easy power, hitting 16 winners to only six unforced errors and creating brilliant angles from the baseline.
“I will be honest, I feel like Aryna did not have the best day,” Andreeva told Sky Sports.
“I feel like I cannot really take all the credit today – I feel like that is not very fair in some ways.
“I’m just happy. I had a goal in front of me to commit no matter what and control the things I can control.”
Belarus’ Sabalenka, by contrast, hit 23 unforced errors and failed to produce a single ace at Indian Wells.
The second seed had taping on her knee but it did not seem to hinder her – this, rather, was a performance similar to some of the collapses she has had this year.
Just as in the French Open quarter-finals, where she lost the third set 6-0, Sabalenka could not find her rhythm or range.
She won only seven points in the second set as Andreeva breezed to an impressive victory.
The result means the winner of the final group match on Friday between Sabalenka and Elina Svitolina will advance to the semi-finals, while Andreeva goes through as the group winner.
Earlier, Svitolina said she “got a bit angry at myself” to inspire a comeback victory over Karolina Muchova and keep her title hopes alive.
Ukraine’s Svitolina lost 6-4 6-0 in her first group match against Andreeva on Monday and made an equally poor start against Muchova.
It looked as though things were getting worse for Svitolina when she took a medical timeout for a right hip injury after losing the first set, before finding herself an early break point down.
However, the sixth seed saved it and then gradually wore down Wimbledon runner-up Muchova to win 3-6 6-2 6-2.
Muchova hit 39 unforced errors throughout the match – the majority of which came from her forehand – as Svitolina ground her down.
The Czech could not recover from giving up an early break in the third set and handed over another with her eighth double fault of the match.
Svitolina made no mistake, with the 32-year-old tipping her head back and roaring in celebration after securing the win.
“I was really angry at myself after the first match and the first set,” Svitolina told Sky Sports.
“Sometimes it [channelling frustration] happens in a good way and sometimes in a bad way.
“I tried to direct my anger towards the ball and I found my game in the end.”
At the Stockholm Open, Britain’s Cameron Norrie fell 7-5 6-3 to world number 10 Taylor Fritz.
Denmark’s Holger Rune reached the quarter-finals of the tournament, a year after he ruptured his Achilles at the same event.
Rune only made his return to the ATP Tour in October after a gruelling 12 months recovering from the injury, but beat Miomir Kecmanovic 2-6 6-1 6-4 to advance.
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