Roger Federer has praised the longevity of Serena Williams as she battles to hold on to the world number one spot at the US Open.
Williams, who moved level with Steffi Graff on 22 grand slam titles with victory at Wimbledon in July, has to reach the final at Flushing Meadows to have a chance of keeping the moniker as the best player in women's tennis with Angelique Kerber in pole position to take over.
The 34-year-old has been at number one since February 2013 and 17-time grand slam champion Federer, who remains at fourth in the ATP rankings at the age of 35 but announced in July he would miss the rest of the year because of a knee injury, has nothing but admiration for the American.
"Well number one I think it's wonderful that she's still playing because for a while it was looking like Serena was not going to stay around the game because it looked like she was interested in other things," Federer told ATP media.
"She was injured [and] she had some health issues, we were worried that Serena was going to leave the game like many others before her around 25 years old, 27, 29, so many girls who left early, so Serena with all her success it would have been so simple for her to do exactly the same.
"But for her to still be playing at 34 now I think she is, I think she is year younger than I am, it's wonderful to see.
"I think it's actually very inspiring for the entire generation that follows her that actually a tennis life on the women's side can also take 20 years and you know you don't have to play every single tournament, but if you pace yourself and you do it professionally [and] stay in good shape like Serena has done so often."