Coco Gauff continued her stellar end to the season with victory over world number one Aryna Sabalenka to set up a WTA Finals showpiece against Zheng Qinwen.
American Gauff had already notched a rare win over Iga Swiatek in the group stage and capitalised on an increasingly wayward performance from Sabalenka.
Gauff beat the Australia and US Open champion 7-6 (7-4) 6-3 to reach the final of the season-ending tournament for the first time.
The 20-year-old has lost just two of her 14 matches since her US Open title defence ended in the quarter-finals in September.
She will face another first-time finalist in Olympic champion Zheng, who beat Wimbledon winner Barbora Krejcikova 6-3 7-5.
“She’s playing great. Playing confident tennis will help me and give me the best shot at winning,” Gauff said.
“Year-end, I consider it as a plus, and even being here is a reward for the season I’ve had.”
This year’s tournament has record prize money of £12m.
Gauff keeps cool as Sabalenka fumes
Sabalenka secured the year-end world number one ranking this week after a stellar season, but she was frustrated, error-prone and annoyed throughout against Gauff.
Her forehand faltered – such a crucial component of her game – with 16 unforced errors coming off it in the first set.
She and Gauff traded breaks early on but Gauff was serene on serve, able to extend the rally in the knowledge that Sabalenka would eventually hit an error.
Despite the chaos, Sabalenka broke for a 6-5 lead and served for the set, but a netted backhand sent it to a tie-break, which Gauff dominated.
After a lengthy bathroom break, Sabalenka started the second set better but another backhand miss hurried Gauff to an early break.
Gauff reeled off four games in a row for a 4-1 lead before Sabalenka eventually retrieved one of the breaks – but she needed eight break points to do that after a mammoth Gauff service game.
Given the erratic nature of the match, a break of the Sabalenka serve in the next game felt inevitable, and it was – as was Gauff being broken when she served for the match at 5-2.
But Sabalenka could not delay the inevitable even on her serve, and a final netted backhand – her 37th unforced error of the night – secured victory for Gauff.
Zheng keeps focus to beat Krejcikova
Crowds have been mixed at the tournament but Zheng has been a consistent draw in Saudi Arabia, with a large number of Chinese fans cheering her on against Krejcikova.
Her game is simple: a big serve, followed up by an equally thumping groundstroke, which initially worked to great effect against Krejcikova.
She moved well around the court, bullying Krejcikova out of rallies and racing to a set and a break lead before the Czech took advantage of a dip in form to force things back on serve.
Zheng grew increasingly frustrated – not helped by her service game twice being held up by a crying baby – and her shots grew wilder, but she kept herself in touching distance of Krejcikova.
A thunderous backhand return of serve gave Zheng break point at 5-5, and she duly took it on a long Krejcikova forehand to serve for the match.
Although Krejcikova again pushed her, with Zheng having to fend off another break point, she ultimately claimed a straight-set win as the Czech went wide with a forehand.
“It feels so special – to be in the final feels unbelievable,” the 22-year-old said.
“It was a hard match for me but it shows I am mentally strong in that moment.”