NEW YORK (AFP) — American teenager Coco Gauff credited controversial Nick Kyrgios with helping her career grow and insisted the Australian does not deserve his "bad guy" image.

Kyrgios, whose natural talent is only equalled by his run-ins with officials, other players and occasionally spectators, has already caused controversy at the US Open.

In his second round match against Benjamin Bonzi, he was warned for spitting and bad language. For his troubles, he picked up a $7,500 fine.

However, Gauff believes Kyrgios does not deserve his bad boy image.

"I remember I first hit with him, he probably doesn't remember this, I was 13 years old at Miami Open. He finished his practice, hit with me for, like, 30 minutes," said Gauff on Friday after reaching the last 16 with victory over US compatriot Madison Keys.

"It's just things like that that stand out for me. That's why I feel like I can never dislike him because constantly when I see him around, he's always saying hello."

Gauff said she hit for an hour with Kyrgios again in Miami the following year, despite him having already practiced for two hours.

Gauff has certainly taken on board any tips she may have received for boosting her service game.

In her second round match against Elena Ruse, she sent down the second fastest ever serve by a woman at the US Open.

It was timed at 128mph (206 km/h) and was just one mile-per-hour behind the fastest serve of men's defending champion and world No. 1 Daniil Medvedev.

"I think in the end it helped me in the long run as a player hitting with him. Nick told me good things about my game," Gauff said.

"At that time I'm like, Nick Kyrgios thinks I'm a good player. You start to believe that and gain confidence in that. Yeah, if he keeps it up, I think he can go far. He can win the tournament."

Wimbledon finalist Kyrgios faces J.J. Wolf of the United States later Friday with a place in the last 16 at stake.