Roger Federer and Rafael Nadal
Roger Federer and Rafael Nadal have been the most successful players on the ATP Tour since the start of 2017. Pictured: Roger Federer of Switzerland talks with Rafael Nadal of Spain during the award ceremony after the Men's singles final match on day eight of 2017 ATP Shanghai Rolex Masters at Qizhong Stadium in Shanghai, China, Oct. 15, 2017. Lintao Zhang/Getty Image

Lleyton Hewitt does not see anyone beating Rafael Nadal at the French Open or Roger Federer at Wimbledon at least for the next few years.

The duo dominated tennis for years until Novak Djokovic entered the fray and successfully challenged their duopoly on the sport. Andy Murray soon joined in as they all formed the Big Four — however, it was the latter two who were slowly challenging each other for the top honors especially in 2016.

With Nadal and Federer seemingly heading past their prime as well as suffering from injuries, with the former not winning a Grand Slam since 2014 and the latter not doing so since 2012, they both experienced a career renaissance last year as they won a combined 13 titles and split the four Grand Slams between themselves, ending the year as the top-two ranked players in the world.

Nadal notably won a record-breaking 10th French Open, making him the only player to win a Grand Slam event in double-digits, while Federer's win at Wimbledon was his eighth, with no other player winning it more times than him.

They continued their resurgence in 2018 as well as Federer won the Australian Open for a second year running while Nadal is currently dominating the clay-court season and is expected to retain his Roland Garros crown.

With the grass events beginning in June where Federer is likely to be a heavy favorite to retain his Wimbledon title, former No. 1 Hewitt finds it hard to see anyone beating the pair at their favored Grand Slam events, and is particularly surprised at how the Swiss legend is still succeeding in the sport.

"I don’t see anyone beating Nadal at Roland Garros in future years," Hewitt said at the Millennium Estoril Open press conference, as per Tennis World USA. "And beating Federer at Wimbledon is also nearly impossible at the moment."

"Federer surprises me more [than Nadal] because is older and had a very deep moment a couple of years ago in his career. A major part of his success is played by his smart way in choosing tournaments to play and that he wants to win."

Hewitt is also impressed at how the duo is regularly beating the standout names in the next generation such as Alexander Zverev, Dominic Thiem and more, all of whom are deemed as successors to the Big Four.

"For some years they were reached by Novak Djokovic and Andy Murray, but they felt again that can win the most important titles," Hewitt added. "It's unbelievable how they keep beating the new generation."

Nadal will take part in the Madrid Open next which takes place from May 7-13. The Spaniard will then feature at the Rome Masters and the French Open. Federer meanwhile, will make his return to action at the Mercedes Cup in Stuttgart, Germany, which takes place from June 11-17.