(CNN) -- Roger Federer and Novak Djokovic have expressed their concern for the future of Rafael Nadal after their great tennis rival pulled out of this month's U.S. Open due to his ongoing knee problems.
The Spaniard withdrew from the season's final grand slam event on Wednesday, when Federer began his preparations at the Cincinnati Masters with a 6-3 6-2 win over Russia's Alex Bogomolov Jr.
"I wrote him and he told me it wasn't looking good at all. I kind of knew," world No. 1 Federer told reporters after setting up a third-round clash with Australian rising star Bernard Tomic.
"But obviously when it's official it's disappointing for tennis, there's no doubt about it. I would love to have him in the draw. In particular 12 days before the Open you figure he might still have time to fix what he has to fix to get ready.
"That's what is sort of scary. There is obviously the whole debate going on if he is going come back for this year. I hope he will. He's definitely got some more weeks off now because of it. I hope in hindsight this is a very smart decision by him. But it's obviously a big blow and disappointing news for the tennis world."
Former No. 1 Nadal won a record seventh French Open title in June after his customary busy clay season, but lost in the second round at Wimbledon and was unable to defend his Olympic singles title at London 2012.
"Tennis is going to lose a little bit because of Rafa not being there and playing, because he's somebody that has made a history of this sport," said U.S. Open champion Djokovic, who began his Cincinnati campaign with a hard-fought 7-6 (7-4) 6-2 win over Andreas Seppi of Italy, having retained his Toronto Masters title on Sunday.
"We all know how good he is and how popular he is. Obviously it's that serious that it takes him out from the tour for the last couple of months. I'm sure that if he was able to perform in the U.S .Open he would come."
Nadal and Djokovic had contested four successive grand slam finals, a run that was broken at Wimbledon when Federer beat Andy Murray to join Pete Sampras on seven titles at the grass major and reclaim the top ranking from the Serb.
Djokovic, last year's Cincinnati runner-up, is seeking to improve his form in his next match against Russian veteran Nikolay Davydenko.
"Really just trying to put another ball more in the court than my opponent was enough to win," he said. "I'm obviously not very satisfied with the performance. I will have to play much better than I did in order to go far."
Murray, who succeeded Nadal as the men's Olympic champion after a revenge win over Federer at Wimbledon this month, also paid tribute to a player he has faced in five grand slam semifinals and two quarterfinals.
"I like Rafa a lot as a friend. I'm disappointed for him," said the world No. 4, who beat American Sam Querrey 6-2 6-4 in his Cincinnati title defense opener, having pulled out in Toronto after his first match due to a knee worry of his own.
"It's obviously tough for him. He's had trouble with his knees in the past. So, you know, I hope he can rest, doesn't come back too early, and gets them fixed so he can get back to playing his best tennis."
The 25-year-old will be hoping for a good run in Cincinnati before continuing his quest for a first grand slam title at Flushing Meadows.
"In the immediate aftermath of big wins or good tournaments, normally you'll feel quite confident and comfortable going for your shots in the important moments and even at the beginning of matches," he said after setting up a third-round clash with France's Jeremy Chardy, who beat Denis Istomin of Uzbekistan 6-4 6-3.
"I'm hoping it helps me not only this week but going into the U.S. Open and big matches in the future. I think it will give me that extra bit of confidence and probably feel a bit calmer going into them."
Fifth seed Tomas Berdych will play 2011 Rookie of the Year Milos Raonic following a straight-forward 6-3 6-0 win over Chinese Taipei's Yen-Hsun Lu.
Canada's Raonic advanced courtesy of a battling 6-7 (6-8) 6-3 6-4 defeat of world No. 42 Marcos Baghdatis.
In the women's tournament, Venus Williams joined sister Serena in the last 16 after coming from behind to beat South Africa's Chanelle Scheepers 2-6 6-3 6-2.
Williams will next face French Open finalist Sara Errani of Italy, who she thrashed in straight sets at the Olympics.
Fourth seed Petra Kvitova, who had a first-round bye after winning her first title this year in Montreal on Monday, defeated Germany's Mona Barthel to earn a clash with China's Peng Shuai.
Montreal runner-up Li Na joined the Czech in round three, where the Chinese ninth sed will play Swedish qualifier Johanna Larsson, while Polish top seed Agnieszka Radwanska and former world No. 1 Caroline Wozniacki also progressed.