Shanghai - Roger Federer warned Novak Djokovic and his other Shanghai Masters rivals on Tuesday that he was in prime condition even as another long season approaches its climax.

The 37-year-old Swiss has been carefully managing his schedule in recent years, cutting out the clay-court segment of the season since 2017 to wring the most out of his ageing body.

It appears to be working and the 20-time Grand Slam winner opens his defence of the Shanghai Masters this week in ominously good shape.

The world number two played in a victorious Laver Cup campaign in Chicago two weeks ago, following his surprise last-16 defeat to the Australian John Millman at the US Open.

"The good thing is after feeling good in practice I played a good Laver Cup, have had also some rest after the US Open," said Federer.

The Swiss great's prime threats in Shanghai are likely to come from world number three Djokovic - who is in fine form - and fourth-ranked Juan Martin del Potro.

World number one Rafael Nadal, beaten by Federer in last year's final of the Masters 1000 showpiece, is absent with injury.

Del Potro, who was hindered by cold and flu in losing Sunday's China Open final to unseeded Nikoloz Basilashvili, said in Beijing last week that he was running on empty.

Not so Federer, it seems, who is seven years older than the Argentine.

"I feel like I'm where I want to be," Federer said.

"I know that this is where I could be playing a lot of tennis, depending on how I play."

Reflecting on his 2018, which included winning the Australian Open and several weeks back at world number one, Federer said: "I have actually been very successful when I played.

"I didn't play many poor matches, to be quite honest. There is only maybe a couple that come to my mind and I have been injury-free for over a year now."

Federer, who is chasing his fourth title this season, will face either Chinese wildcard Zhang Ze or feisty Russian Daniil Medvedev in his opener.