Serena Williams said she was “nowhere near” as good as the 15-year-old Coco Gauff when she was that age as the American great reached the Australian Open third round on Wednesday, despite being far from her best.

The 38-year-old, one short of Australian Margaret Court’s all-time record of 24 Grand Slam titles, fought off a stubborn Tamara Zidansek 6-2, 6-3.

Serena faces 27th seed Wang Qiang of China next and could meet fellow-American Coco in the quarters.

The former World No. 1 and Coco — touted as a possible long-term successor to Serena at the top of women’s tennis — spent some of the off-season together.

Serena, sporting a pair of koalas painted on her nails in light of the devastation of Australia’s deadly bushfires, was mightily impressed by what she saw in “fighter” Coco.

“She’s just impressive all-round, from her personality to the way she plays,” said Serena, the top-earning female athlete in the world. “I think it’s just all super-impressive.

“I was nowhere near her level at 15 either on the court or off the court, not even close.

“I know kids are growing up different nowadays, so I’m not sure. But I was nowhere near as smart and eloquent as she is. It’s nice to see.”

Coco defeated Serena’s elder sister Venus out in the first round in Melbourne, having done the same at Wimbledon last year.

Venus turns 40 in June, but Serena said out of the two sisters, she will be the first to retire.

“Venus will outplay me, for sure, she loves tennis a lot,” said Serena, who is in the twilight of her career but has given no hint that she plans to quit any time soon. “She works hard on it, just gets unlucky sometimes health-wise obviously.”

Coco survived a three-set thriller against Sorana Cirstea to book a third-round showdown with defending champion Naomi Osaka for the second Grand Slam in a row.

“You really made me believe,” Coco told fans at the Melbourne Arena.

Coco, the youngest player in the draw, will next face Japan’s Osaka, who defeated her 6-3, 6-0 in the third round of last year’s US Open.

That encounter ended with Osaka drawing praise for allowing an emotional Coco to thank her home fans in a post-match interview.

“Yeah, it was definitely a good moment I think for both of us, especially me,” Coco told reporters.

“But I think more just for the people watching, the little girls and little boys who can kind of see what sportsmanship is really.

“When it’s all said and done, we still look at each other with respect and the same.”

Coco expects to be “less nervous” against Osaka this time around as she looks to make the fourth round at a major for the second time after her Grand Slam debut at Wimbledon last year.

“We’re both familiar with each other's games. She plays really aggressive. This time coming in, I’m going to be more aggressive,” she said.

Coco found herself playing catch-up against the composed and experienced Cirstea as she struggled with errors off her forehand.

But after taking the first set Cirstea failed to maintain her standards in the second, allowing Gauff to capture an early break and force the match to a decider.

The final set was a gripping, evenly-matched affair as Gauff battled from a break down to get back on serve at 2-3.