HONG KONG - 18-year-old Dayana Yastremska has forged into her second career WTA quarterfinal at the Prudential Hong Kong Tennis Open with a 6-3, 6-3 defeat of Zheng Saisai in an all-unseeded second-round tilt.

The teenager previously made the last eight in Istanbul last April as a wildcard ranked No.420 - and today, she returned to that stage by avenging a first-round exit at the hands of Zheng last week in Beijing in a match delayed for several hours because of rain.

"The day was so long!" said Yastremska afterwards. "I woke up very early and I was very nervous because I lost to her in Beijing, I knew she was really good."

The Chinese player had been too crafty for Yastremska in that match, winning 6-4, 6-3 to maintain her still-perfect record in Asian swing openers this season, and she picked up where she left off on the very first point today, drawing her opponent into net with a short slice before dinking a pass past her.

But Yastremska's rapid rise over the past two years has been indicative of a player who is learning the ropes of the professional tour quickly, and the Ukrainian quickly gave notice that this week would be a very different story. Tempering her power with a greater amount of steadiness, the World No.102 demonstrated greater patience in rallies - but was no less lethal, as the booming backhand winner down the line to break for 3-1 showed.

Winners began to flow from the Yastremska forehand as well as she stormed to a 5-1 lead, punishing Zheng's second serve relentlessly to capture the second break while saving two break points against her. Closing out the opening act would not be straightforward - a sudden cascade of errors led to the youngster losing eight of the next nine points - but, unfazed, rediscovered both her first serve and her groundstroke range to serve it out at the second opportunity.

"I felt really good on court - even though I was feeling very weak before the match," revealed Yastremska. "I don't know why - just like it's too late and I'm not used to playing that late. And after I went on court I didn't see any ball, the clouds were very dark and I'm not really good at playing under the lights. But I wasn't really thinking about that, I was just trying to fight for each ball and I found my game from the first shot."

Dominating with her forehand, Yastremska came through an exchange of breaks at the start of the second set to take a 3-1 lead - but the Nanchang runner-up still had some tricks up her sleeve. Showcasing some brilliant defence, Zheng continued to test the 2016 Wimbledon junior finalist's consistency - and in the eighth game, came up with the shot of the day, a precisely judged dropshot at full stretch off a full-blast Yastremska forehand that landed for a clean winner.

But the youngest player in the Hong Kong draw was supremely focused in riding the ebbs and flows of the scoreline today: on each of the three occasions the World No.58 broke her serve, Yastremska would immediately break back, and that dropshot simply inspired her to raise her level even higher. A series of backhand bullets confirmed the 5-3 lead, and some ferocious returning erased Zheng's 40-0 lead on serve in the next game. Though Yastremska, refusing to let up on her aggression, missed by inches on her first two match points, Zheng's third double fault of the day sealed the result after one hour and 24 minutes on a fourth.

Yastremska, who became the first player born in this millennium to crack the Top 100 in July, had entered Hong Kong with just a 1-6 record in WTA main draws this season - compared to a 34-11 record in ITF events and WTA qualifying. But her 11th career Top 100 victory indicates that she is increasingly thriving at this level - and a quarterfinal against Kristina Kucova, conqueror of No.3 seed Jelena Ostapenko, will be a good opportunity to notch up a first trip to the last four.

"It's the end of the season, and I'm going to give my all - and then I can rest!" declared Yastremska.