SHENZHEN, China - Last-minute alternate Kiki Bertens took full advantage of her good fortune at the Shiseido WTA Finals Shenzhen, scoring her first win over No.1 seed Ashleigh Barty at the sixth attempt 3-6, 6-3, 6-4 from a set and a break down in two hours and nine minutes.

The Dutchwoman's quest to qualify for her second consecutive WTA Finals has been an arduous one taking in seven consecutive weeks of play across two continents since the US Open - and despite that, it had seemingly ended in failure as she finished ninth in the Porsche Race to Shenzhen. Indeed, Barty had single-handedly kept Bertens out of the year-end finale by the narrowest of margins a month ago, fending off a match point to take their Beijing semifinal - a win which would have sealed Bertens' place in the elite eight.

But as the first alternate this week, Bertens was the last-minute substitute after No.3 seed Naomi Osaka withdrew from the tournament due to a shoulder injury - and immediately took her revenge, preventing Barty from guaranteeing her spot in the semifinals, which a win today would have sealed for the Australian, while keeping her own chances of progressing out of the Red Group alive.

"As the match was continuing, I felt better and better," said Bertens afterwards about how she had managed to battle through fatigue for the win. "The movement was getting a bit better. I was starting to feel the court. I could play a little bit more aggressive, come to the net."

She had not expected to be called into action today - "I was OK sitting on the couch," Bertens laughed about her expectations as an alternate - though was given enough notice to get in one last nap before making her way on court. "I was really tired," she said, to the surprise of few. "But then yesterday I just hit for five or 10 minutes. I just rested completely. I think today I felt already much better. Of course, it's not like after one day that you're completely back in full energy."

The first set was characterized by a feast-or-famine approach from the service line for Roland Garros champion Barty, who opened the match with a love hold in barely a minute and closed out the stanza with two more. In between, though, both players would come up with their best tennis on return, resulting in a sequence of five consecutive breaks. Still, it was Barty who led throughout, maintaining her lead with several highlight reel shots - including twice taking break points by driving through her backhand instead of her preferred slice, rewarded both times by nailing brilliantly angled winners.

Bertens, playing for an eighth consecutive week since the US Open, was able to keep Barty on her toes with some fine defensive work and net finishes of her own - but, landing just 43% of her first serves and racking up 14 unforced errors to seven winners, was only able to hold once, and never quite managed to catch up with the World No.1.

In the second set, though, it was Barty who became significantly more mistake-prone, racking up 15 unforced errors as her first serve percentage plummeted from 56% to 41%. Despite grabbing the first break of the set for 2-1, the 23-year-old immediately conceded it with three straight poor forehand errors - and this time, Bertens was able to take advantage.

Rushing the net to shorten points whenever possible, the St Petersburg and Madrid champion was able to show off both her heavy power on the smash and her touch on a series of exquisite stop-volleys. Indeed, Bertens' finesse would be close to its best during this passage of play, with a wickedly backspun counter to a Barty dropshot leaving the Sydney and Beijing runner-up flailing.

As Bertens' level rose, Barty fell away dramatically. From a break up at 3-2, a cascade of unforced errors from the Australian Open quarterfinalist, eventually tallying 41 to 30 winners, led to a run of eight straight games for Bertens, with the opening stages of the deciding set proving a particularly one-sided stretch: the 27-year-old rattled off 17 out of 20 points as she quickly built a dominant 4-0 lead.

Bertens' run of games had begun after a key on-court visit from coach Elise Tamaela - which served more as a reminder than a new tactic. "She was like, You just start to play better and better, you start feeling the court better, you're moving better," revealed Bertens. "Just try to stay and play aggressive. Come to the net, just go for your shots, move from the legs. I knew I had to do all those things, but I was not really feeling the body. I think I did it pretty well. Just keep coming into the net. I hit some great volleys today."

Barty would belatedly come alive at this point to stem the tide, making for a ather more gripping dénouement to the match than the previous passage of play had indicated. The Cincinnati and Wuhan semifinalist faced three points to fall behind a 0-5 triple break, and three more to go down 1-5 - but saved them all, and over the course of 11 deuces and two games began to slowly peg Bertens back.

With Barty nailing her overheads and able to deploy her variety with accuracy once again, a remarkable comeback loomed - but, battening down the hatches and focusing on the basics, Bertens made the full use of her insurance break. Landing her first serves and heavy forehands, the Zhuhai runner-up posted two calm holds to hold off the resurgent Barty, reaching match point with an athletic high backhand volley and sealing victory with a simple one-two punch.