The Swiss veteran has spent more time at the top of the rankings than any other men's player - 310 weeks including 237 in succession.

Speaking in Melbourne before next week's Australian Open, Federer said he was a big fan of Barty, who finished 2019 as the top-ranked women's player.

"What a year she's had; it was a pleasure watching her play," Federer, the world No.3, said.

"I like how she mixes it up, she uses the slice to great affect and she's got variety."

Federer said, while some felt the top ranking carried extra pressure, he felt it relaxed him and urged French Open champion and WTA finals title-holder Barty to embrace the achievement.

Barty will start as a favourite at Melbourne Park, with her previous best showing coming last year with a quarter-final berth.

"She can have a different mindset," 20-time grand slam champion Federer said.

"It can relax you and you can really explore all of your potential and that's what happened to me when I won my first major, and I broke through as world No.1 after that.

"I thought, 'Well now I'm the guy to beat and I prefer to be in this position rather than being a contender'.

"I hope that Ash is also going to see it that way."

The 38-year-old Federer, who is gunning for his record-equalling seventh Open title after a fourth-round exit last year, had also been playing close attention to Australia's top two male players in Alex de Minaur and Nick Kyrgios.

He said that world No.29 Kyrgios had the game to win his first grand slam if he could get his mind right.

"Nick doesn't need to improve a whole lot in his game, he knows he can beat everybody," Federer said.

"With Nick we know it's elsewhere - it's his mind - how much can he do, does he want to do?"

Federer said he hoped that Kyrgios would scale back his events in 2020 but be at 100 per cent in each one.

Federer watched as de Minaur pushed Rafael Nadal before succumbing in three sets in their ATP Cup semi-final clash and said he was expecting the 20-year-old to have a big Open.

"Last year was a breakthrough season for Alex, winning three events, and it showed that in the bigger stages he was ready to play," Federer said.

"Home pressure isn't an issue for him so I think we'll see much more from Alex."

Australian Associated Press