U.S. Open champion Coco Gauff overcame hard-hitting Czech teenager Barbara Fruhvirtova 6-3, 6-0 on Thursday to extend her winning the start to the year and the defense of her title at the Auckland Tennis Classic.

The 16-year-old Fruhvirtova came out hitting harder than any player at the tournament so far: generating enormous power off both hands.

She troubled Gauff at first and broke the American’s serve to love in the third game of the first set.

Gauff immediately broke back in a game of 14 points to level the set at 2-2. After saving a break point in her next service game, Gauff broke again to lead 4-2.

She served for the first set at 5-3 and had to save a break point before clinching the set in 40 minutes. Fruhvirtova tested Gauff with more high top-spin but Gauff came through and finished the set with an ace.

Gauff immediately broke Fruhvirtova in the first game of the second set as she began to find more depth in her returns. She also broke in third and fifth games and held her serve with confidence, highlighting her second service game with a 190 kph (118 mph) ace.

Gauff had to save two break points as she served for the match at 5-0 but managed to allay any threat with her sixth ace.

“I thought I played well,” Gauff said. “She started really well and I was able to raise my level. I thought I served and I did better on my returns.”

Fruhvirtova came into the match with a WTA ranking of 117.

“I thought she played a good match,” Gauff said. “I’ve known her since she was 10 years old. We used be in France, training at the same academy. Her and her sister (Brenda), we grew up playing or training beside each other and it’s great to see her on tour. I’m sure she’s going to go even further.”

Gauff now will play eighth-seeded Varvara Gracheva of France in the quarterfinals on Friday. Gracheva beat Lulu Sun of Switzerland 6-3, 6-4.