Petra Kvitova conceded she had no answer as Ashleigh Barty sealed her place in the semi-finals of the elite season-ending championships with a decisive straight-sets win over the dual Wimbledon champion in China.
Rebounding from her last-up pool loss to Kiki Bertens, Barty delivered the most impressive serving display of the WTA Finals thus far to top the Red Group in Shenzhen with a 2-1 record for the week.
Belinda Bencic claimed the second semi-final spot from Barty's group when an ailing Bertens called it quits while trailing the Swiss 7-5 6-1 in Thursday night's second match in Shenzhen.
A replacement for injured world No.3 Naomi Osaka, Bertens said she was physically and emotionally spent after rallying from a set and a service break down to beat Barty on Tuesday night.
That victory was the Dutchwoman's tour-best 55th win of the season and came only two days after she had contested the final in Zhuhai.
"I gave absolutely everything this year and this week. I'm proud of that," Bertens said after being in tears before stopping on medical advice, having failed to serve out the opening set against Bencic from 5-3 up.
"Like five weeks ago, I said to myself, 'You're completely dead'. I guess now I'm almost there.
"It was really tough, but I didn't feel well on court. I had some problems with my stomach. Just the energy was not there. A little bit dizzy.
"Yeah, just too much to continue."
Showing why she's already wrapped up the year-end top ranking, Barty had no such troubles backing up.
The first eight matches of the tournament had produced an extraordinary 65 service breaks - at an average of one every three games - on the notoriously slow purple indoor court.
But, clutch under pressure, Barty was unbroken in downing Kvitova for the third straight time since losing to the Czech world No.6 in this year's Australian Open quarter-finals.
"I felt like I executed really well tonight," Barty said.
"Playing against Petra is probably one of my favourite things to do if I'm being honest.
"Win or lose, it's going to be a great match. It's going to be played in the right spirit. I just enjoy the challenge of playing against Petra. She really does bring out the best in me.
"Each time I've played Petra, I've become a better player having learned what's happened during that match."
The French Open champion is a vastly different player to the one that lost to Kvitova in their first four encounters.
"It was pretty difficult from the baseline to play with her rallies," Kvitova said.
"I just couldn't find probably the rhythm. She was just better, for sure."
Barty will play either world No.2 Karolina Pliskova or Wimbledon champion Simona Halep - who clash on Friday night - for a spot in the title decider.
Bencic's reward for making the semi-finals on debut, like Barty, is a showdown with defending champion Elina Svitolina, the Purple Group winner after not conceding a set in wins over Pliskova and Halep.
Svitolina's last round-robin match on Friday is against American Sofia Kenin, the replacement for US Open champion Bianca Andreescu, who has withdrawn with a knee injury sustained against Pliskova on Wednesday.
