TOKYO — Teenage tennis sensation Cori Gauff's first coach Sly Black has backed the American to become the world's best.

Gauff, 15, beat five-times champion Venus Williams and 2017 semifinalist Magdalena Rybarikova in her first two rounds and will play in the third round at Wimbledon later on Friday.

Black, who has also worked with fellow American Sloane Stephens, remembers the time when Gauff's family brought her to his Florida base as a six-year-old. Even then, her talent was obvious.

"I predicted she was the best 10-year-old tennis player in the history of the sport at that moment," Black told Reuters through Skype from Vietnam, where he is currently coaching.

"I have seen Martina Hingis, I have seen Jennifer Capriati at 12-years-old but Cori at 10-years-old was, in my mind, the best 10-year-old tennis player to ever play this sport."

Gauff, also known as Coco, has played a fearless brand of tennis at Wimbledon, showing a strength of character that belies her age. Mental strength is what sets her apart from the other youngsters Black has worked with down the years.

"First of all, she had the physical ability, that's the first thing, then her mental toughness was way ahead of her age," said Black, who continued working with Gauff until 2015.

"I have coached many girls at the same age, at that age — between eight and 15-years-old — and Cori's mental ability was just as good as any top 15-year-old girl I have ever seen. At that age she was well ahead of her time."

Gauff has won both her Wimbledon matches in straight sets to set up a meeting with Polona Hercog in round three. Despite a slight waver following her win over Williams, Gauff has taken the attention in her stride so far at the All England Club.

Black has backed her to maintain her focus. "I have seen her do things at a young age or win games at a young age that she is not supposed to win," said Black, who witnessed Gauff, at the age 13, become the youngest to reach the US Open girl's final.

"If there is anybody who is going to be able to maintain or keep up with the pressure that is happening right now with her is Cori."

Back when Gauff was just 10-years-old, Black predicted she would be a top 10 player by the age of 16 and the world number one at 17. He stood by that prediction and believes the young American might even be ahead of schedule.

"She has arrived and she has arrived in style and is there with the best," said Black. "Everyone knows who she is and she is on target right now, for sure, to become the best tennis player in the world." — Reuters