Toni Nadal expects a grand slam title to prove beyond Roger Federer in 2019.
The Swiss extended his all-time men's record to 20 slam victories by winning the Australian Open in January but suffered some unexpected losses during the rest of the season, finishing it ranked third.
Nadal, the long-time former coach of his nephew Rafael, believes the physical demands of best-of-five-set test are likely to put the majors out of reach for 37-year-old Federer.
Writing in his column in El Pais, Nadal said: "What can we expect from 2019? I find it hard to see Federer lifting another grand slam cup.
"Obviously, I do not say it for his game, although I do say it for the toughness of the five-set tournaments. I have to confess that I said that on other occasions and the Swiss, repeatedly, surprised me."
Rafael Nadal is to undergo ankle surgery after revealing he is out of the ATP Finals in London because of injury.
The Spaniard pulled out of the Paris Masters with an abdominal injury and after failing to recover in time, has now withdrawn from next week's event.
He was scheduled to have a procedure to remove a floating body from his ankle on Monday in a bid to be fit for the start of the new season.
In a post on his official Facebook page, Nadal said: "It has been a complicated year, very good at the tennis level when I was able to play, and at the same time very bad as far as injuries are concerned.
"I have done everything possible to reach the end of the season in good condition, both in Paris and London, doing things well and I really wanted to play.
"Unfortunately, I had the abdominal problem in Paris last week and, in addition, I have a free body in the ankle joint that has to be removed in the operating room today.
Rafael Nadal has announced he will not be taking part in next month's China Open and Shanghai Masters due to his knee issue.
The world number one was forced to retire from his US Open semi-final against Juan Martin Del Potro earlier this month because of the problem in his right knee.
And he wrote on Twitter on Wednesday: "This past Monday I was in Barcelona looking with the doctors the situation of my knee.
"While the discomfort in my knee is nothing new, we decided together with my medical and technical team not to participate in the Asian tour to recover the knee in the way we have always done."
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Rafael Nadal believes now is the moment to step up the defence of his US Open title after another tough battle against Nikoloz Basilashvili.
After battling for nearly four and a half hours to beat Karen Khachanov on Friday, Nadal was again pushed beyond the three-hour mark before defeating his Georgian opponent 6-3 6-3 6-7 (6/8) 6-4.
Basilashvili, ranked 37, had won just a single game against Nadal in their only previous meeting at the French Open last year but played superbly here, especially in the third-set tie-break.
Next up for Nadal is a quarter-final against ninth seed Dominic Thiem, who is the only player to beat him on clay for the last two seasons and who ended the run of last year's beaten finalist Kevin Anderson on Sunday.
Nadal defeated Thiem in the French Open final but has a lot of respect for the Austrian. This will be their first meeting on hard courts after 10 matches on clay.
"He's a fantastic player," said Nadal. "He's a very powerful player. He's a great guy. I have a very good relationship with him.
"It's going to be a tough one. He knows how to play these kind of matches. I need to play my best match of the tournament if I want to keep having chances to stay in the tournament.
"Now is the moment to make the next step, step forward, play more aggressive. I did a lot of things well during the whole season. It's the moment to make it happen again. I hope to be ready to make that happen."
Rafael Nadal was in ominous form as he dispatched Vasek Pospisil to set up a third-round clash with Karen Khachanov at the US Open.
The defending champion took exactly two hours to clinch a 6-3 6-4 6-2 victory under the lights on Arthur Ashe Stadium, with the only moment of alarm coming in the second set when he dropped serve to trail 4-2.
But Nadal broke straight back and a run of four games in a row gave him the second set, with the third largely one-way traffic.
Nadal has played both his matches so far at night, when there is some respite from the searingly-hot conditions, although the humidity means temperatures have still been pushing 30C.
The world number one expects the level of tennis on show to go up as the temperature goes down, saying: "The good thing is I think things are going to improve the next couple of days.
"We'll be able to see, I think, better tennis than we saw the first couple of days of the US Open because playing under those conditions is, first thing, not healthy and, second thing, is not good for anybody: not good for the fans, not good for the players.
"The show is a little bit worse under these conditions."
World number one Rafael Nadal has pulled out of this week's Cincinnati Masters.
The 32-year-old Spaniard, who won the title in 2013, feels he needs a rest following Sunday's victory at the Rogers Cup in Toronto.
"I am very sorry to announce that I won't be playing in Cincinnati this year," Nadal wrote on Twitter.
"No other reason than personally taking care of my body and trying to keep as healthy as I feel now.
"I am very thankful to my friend Andre Silva, tournament director of the Cincinnati tournament, who after speaking to him on the phone understood what I said and understands the situation.
"I am sure the tournament will be a success and I wish him and his team all the best. I am also sad for those amazing fans in Cincy and around who always gave me great support.
"I am sure I'll see them next year and I am sorry I won't be with them this year. Thanks for all the support as always."
Nadal, the 17-time grand slam winner, collected his 80th career title on Sunday by beating Greek youngster Stefanos Tsitsipas 6-2 7-6 (7/4).
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Rafael Nadal has withdrawn from the Fever-Tree Championships at Queen's Club, three days after winning his 11th French Open title.
It is the third consecutive year the world number one has entered the tournament, which begins on Monday, only to pull out before the start.
Nadal said: "Queen's is a great event, I have happy memories of winning the title in 2008 and I wanted to come back this year. But it has been a very long clay-court season for me with great results.
"I would like to say sorry to the tournament organisers and most of all to the fans that were hoping to see me play, but I have spoken to my doctors and I need to listen to what my body is telling me."
Nadal last played at Queen's Club in 2015 and will once again go into Wimbledon without any official grass-court matches. He has struggled at SW19 in recent years, failing to progress beyond the fourth round since his last final appearance in 2011.
Queen's organisers announced the strongest line-up in the tournament's history and will now wait to hear whether Andy Murray will follow Nadal in withdrawing.
The two-time Wimbledon champion is expected to decide in the next couple of days whether to make his comeback from hip surgery in West Kensington.