Rafael Nadal, Novak Djokovic and Roger Federer will face a stronger challenge to their dominance than ever this season, warned two players aiming to lead the charge.
Dominic Thiem, the world No 4, and sixth-ranked Stefanos Tsitsipas both demonstrated in 2019 why they - along with world No 5 Daniil Medvedev - will be at the head of the chasing pack in 2020.
Thiem won an ATP Tour-leading five titles last season - the same number as Djokovic - which included a Masters 1000, while reaching a second successive French Open final as well as the final of the ATP Finals.
Tsitsipas, 21, claimed the biggest ATP title of the year when he beat Thiem to win the season-ending Finals in London at the end a campaign that featured wins over Nadal, Federer and Djokovic - becoming the youngest player to beat the 'Big Three' in the same season.
Medvedev, meanwhile, reached a tour-leading nine finals, including his first grand slam final at the US Open, won four titles - two of which were Masters 1000s - and recorded successive victories over Djokovic.
While progress was evidently made in closing the gap at the top, grand slam success remained elusive - with Djokovic winning the Australian Open and Wimbledon and Nadal victorious at Roland Garros and the US Open - and the top-three spots in the ATP rankings stayed untouched.
However, as a new season begins at the inaugural ATP Cup in Australia, optimism is high that can change over the coming 12 months.
"I think we challenged them already a lot. We also beat them especially on Masters 1000 stages and the ATP Finals," said Thiem, who begins his ATP Cup campaign on Friday against Borna Coric as Austria face Croatia in Sydney.
"I think the last stage we have to conquer is the grand slam stage, and I really think we'll see a new grand slam champion in 2020. I really hope that it's me, but you cannot guarantee anything. There are also many other very strong guys who can achieve that.
"Maybe it's also not happening and 'Big Three' are taking all four, but we are trying everything to achieve top position, the biggest titles, and grand slam titles."
Tsitsipas is similarly confident of a grand slam breakthrough this season. While competing at the Mubadala World Tennis Championship in Abu Dhabi last month - where he lost to Nadal in the final - the Greek player made clear his aims for 2020 included breaking into the top three and winning a grand slam title.
Now in Australia, Tsitsipas reiterated those ambitions and said he will continue to learn from the three leading players every time he faces them on court. Tsitsipas gets his ATP Cup underway against Canada's Dennis Shapovalov.
"They are still there, they are doing really well," Tsitsipas said. "For me, it's very challenging to go out there and compete against them.
"I feel like there's something extra to chase after when I play against them, plus I get to learn a lot when I play against them. I compare them to me to see what works for them and what should I work on more, and they're just super humans in a way.
"But I'm trying to get there myself and I'm going to keep trying to do my best because I feel like I can surpass them, I can be better. It's just a matter of time, patience, and dedication on what I do."
The ATP Cup is a new team tournament that will kickstart the tennis season for the world's leading male players. Divided into six groups of four, nations qualify based on the ranking of their leading available player.
The winners of each group, plus the two leading runners-up, will progress to the quarter-finals, with each tie decided after two singles matches and one doubles match - similar to the Davis Cup format.
However, unlike the Davis Cup, ATP ranking points are also up for grabs as well as total prize money of $15 million (Dh55.10m).