Roger Federer has fired an ominous warning that US Open champion Novak Djokovic is still short of his very top form.
Former world number one Djokovic ended a two-year wait for grand slam glory by winning Wimbledon in July and added a 14th major title at Flushing Meadows this month.
That latter triumph saw him move level with Pete Sampras on the all-time list of male grand slam triumphs, the pair ranked third behind Federer (20) and Rafael Nadal (17).
But Federer believes that Djokovic still has more gears to reach having recovered from the injury and form woes that saw him lose his grip atop the rankings.
"I think that there's more to come from Novak," he said at an event held by the Andy Roddick Foundation in Austin.
"I don't even think he's playing his absolute best since his injury. He's playing very, very well.
"In his best years, a few years ago, when he was almost, virtually, unbeatable I still think he has a little bit extra to go, so I think it's very encouraging for him."
Federer also welcomed the added competition from the likes of Andy Murray, Kei Nishikori, Stan Wawrinka and Milos Raonic who have all had their own injury woes.
"It's nice to see Wawrinka back, Raonic back and Nishikori and Murray, all these guys who were injured a lot in recent years," he added.
"I actually think men's tennis is doing great right now."
Comparing Djokovic and Sampras, Federer added: "Very different. They play very different.
"I think Novak's more outgoing, Pete was more in his shell. One would come to the net, one is an aggressive baseliner.
"What an accomplishment it is to equal Pete's record. That record seemed so far away for all of us, you know, for so many years."
