STRASBOURG, France - Teenage qualifier Marta Kostyuk has reached her maiden WTA quarterfinal at the Internationaux de Strasbourg with a 6-3, 6-1 upset of No.7 seed Zheng Saisai in one hour and 13 minutes.

Gallery: Sizzling in Strasbourg: From Rezai to Pavlyuchenkova

Having started 2019 at a career-high ranking of World No.116, the 16-year-old had plummeted to No.320 just three weeks ago following a spell of health and off-court issues: after pushing Caroline Garcia to three sets in Stuttgart last year, Kostyuk's record for the remainder of the 2018 season was just 4-9. But the past month has seen the Ukrainian, who parted ways with former coach Dimitriy Brichek in April and is now working with Benjamin Ebrahimzadeh, bounce back strongly, defeating fellow teenage phenomenon Amanda Anisimova en route to qualifying for Madrid.

Read more: Marta Kostyuk: 'I was the first who broke through and the first who fell'

Now back up to No.264, Kostyuk's fourth career Top 50 win will see her rise to the brink of re-entering the Top 200, sealing her spot in Wimbledon qualifying.

In an opening set of subtle strategic adjustments, Kostyuk started confidently, showing off both her power off the ground and her instincts at net as she took a 2-0 lead. Zheng would hit back with canny use of angles and accurate passing shots to reel off three games of her own, breaking the teenager twice to move up 3-2 - aided by consecutive double faults from Kostyuk in a rare display of carelessness - but Kostyuk wasted little time in wresting momentum back, punishing the Chinese player's serve with booming backhand returns.

Once again, too, Kostyuk would show off a natural all-court game in addition to first-strike power. A dropshot combined with an athletic, leaping volley to hold for 4-3 was a particular highlight, and the teenager would find excellent winning angles off both wings to seal each of the next two games for the set.

The initial stages of the second set were a race forwards as both players sought to take the net away from each other. Although Zheng found some success with this tactic at first, Kostyuk would soon get the upper hand - both at net and, showing off some more phenomenal shotmaking from the backhand wing, in keeping her more experienced opponent pinned to the baseline as well.

Read more: Beyond Svitolina: Behind the scenes of Ukraine's surge in tennis talent

Going from strength to strength, Kostyuk smothered Zheng's options by excelling in every aspect of the game: displaying the new maturity she has discussed in interviews, the youngster was patient when she needed to engage in extended rallies, rarely going for glory shots too early, but bold in striking out decisively when the time was right. Kostyuk was particularly impressive in upping her power on big points, clubbing breathtaking backhand winners to break Zheng for 3-1, to dig herself out of a 0-30 hole at 4-1, and finally to convert her second match point.

Up next in the quarterfinals will be either a rematch with No.4 seed Caroline Garcia or a clash with Rebecca Peterson as Kostyuk bids to reach her first semifinal.