STUTTGART, Germany -- No.6 seed Kiki Bertens of the Netherlands used a stellar serving display to come back from a set down and outlast Switzerland’s Belinda Bencic in the second round of the Porsche Tennis Grand Prix on Thursday, 4-6, 6-3, 6-4.

"For sure, it was a fight," Bertens told the press, after her victory. "I’m just really happy that I could manage to turn it around and, of course, my serve helped me a lot today."

Big-hitting Bertens, who has the second-most aces on the season behind only Karolina Pliskova, fired 20 on the day as she improved her undefeated record against Bencic to 3-0 following her one-hour and 59-minute victory. Bertens moved to 3-3 against Top 20 players in 2019, while denying World No.20 Bencic an astounding seventh Top 10 win of the season.

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World No.7 Bertens, into the Stuttgart quarterfinals for the first time, could convert just three of her 13 break points. However, her 41 winners, nearly half of which being her 20 aces, were enough to help her eke past Bencic in the end.

"We worked on [the serve] a lot during the off-season, just hit a lot of balls, just trying to the rhythm a little bit faster," Bertens admitted. "Also physically we worked a lot just to get the rotation a little bit more over the serve and coming a little bit more from the legs. So, I’m really happy that it’s paying off."

Bertens moves into the final eight in Stuttgart, where she will face the winner of the all-German nightcap between former Top 10 player Andrea Petkovic and No.5 seed Angelique Kerber, the Stuttgart champion in 2015 and 2016.

"I think it’s going to be a nice German battle out there," said Bertens. "For me, it doesn’t matter who’s going to win. I’m just really happy to get through today, I think I was a little bit lucky and hopefully I can play better tomorrow."

In the first set, Bencic wriggled her way out of a tricky early service game, as the Swiss player had to stave off five break points to hold for 3-3. Meanwhile, Bertens was cruising on serve at this point, blasting through a second consecutive easy service game to hold on at 4-3.

But Bencic cracked through the imposing power game of Bertens at 4-4, taking advantage after the Dutchwoman double faulted on game point. At deuce, Bencic hit a strong return to force a long reply from Bertens, bringing up break point. There, a long backhand from Bertens gave Bencic the only break of the set, en route to the one-set lead.

Bertens found herself in big trouble after her unforced errors persisted, causing a loss of serve in the first game of the second set. But the Dutchwoman suddenly rebounded, blasting a big return to immediately break back, and after holding at love for 2-1 with an ace, Bertens had new life.

More aces from the stupendous server pulled Bertens through uncomplicated service games up to 4-3, where the No.6 seed took command of the set after Bencic dropped serve with a forehand error on break point. Serving to level the match, Bertens reached set point with another ace, and got to one set apiece after claiming the next point.

Bencic had a chance to take an early lead in the decider: the Swiss star held a break point in the second game, and she had gone 2-for-2 on those chances in that match up to that juncture. But it was here that the serving of Bertens became absolutely crucial, as she fired two consecutive aces to get to game point, ultimately holding for 1-1.

From that point forward, Bertens was unstoppable on serve, and once Bencic lost her own serve at 2-2 with a double fault, the Dutchwoman inexorably moved towards the comeback victory. A love service hold at 5-3 sealed the deal, and clay-court maven Bertens notched a quality win over one of the season’s upset experts.