Naomi Osaka has won hearts all over again.

The defending champion consoled 15-year-old American Coco Gauff after beating her 6-3 6-0 in the third round of the US Open.

"For me a definition of an athlete is someone who treats you as their worst enemy on the court but after they treat you like you're their best friend. That's what she did," Gauff said.

It was not the first time Osaka has shown compassion in victory and after a heart-warming embrace at the net, she invited a tearful Gauff to speak to the crowd in a post-match interview.

"She [Osaka] told me that I did amazing and 'good luck' and asked if I could do the on-court interview with her. I said 'no' because I knew I would cry the whole time, but she encouraged me to do it," said Gauff, competing in only her second Grand Slam singles main draw.

"It was amazing, she did amazing and I am going to learn a lot from this match. She has been so sweet to me, so thank you for this."

Osaka, 21, was holding back tears herself when she then spoke directly to Gauff's box, before apologising for playing so well.

"You guys raised an amazing player," said the Japanese world number one. "I used to see you guys training in the same place as us. The both of us made it and are working as hard as we can. I think this is the most focused I have been since Australia.

Turning back to Gauff, she added: "I am sorry for playing you in this mentality. It was super fun!"

Tearful - but a contrast to last year

Japanese top seed Osaka announced herself to the tennis world when she won the US Open last year after an angry Serena Williams accused the umpire of being a "thief" in some of the most dramatic scenes at a Grand Slam final.

Williams was given a game penalty for her outburst, which followed racquet smashing and another code violation as Osaka won 6-2 6-4.

"I'm sorry it had to end like this," a crying Osaka said during the trophy presentation.

Fast-forward 12 months and Osaka was once again up against the home favourite on Arthur Ashe. But this time the tears were different.

Osaka, though still only 21, was the senior - showing grace and maturity as she offered advice and comfort to Gauff - but could not hide her emotions when speaking to the teenager's family.

And remaining humble, Osaka laughed when it was suggested she was a "mentor" before hugging Gauff as the American left the court.

It was a moment which American 11th seed Sloane Stephens said is "what tennis should be about".

'Proud to have Osaka as number one'