Novak Djokovic already laid a marker for his opponents in the ATP World Tour Finals, by thumping Kei Nishikori in his first match, but Roger Federer is one man who believes the world number one is far from unbeatable.

Djokovic has been virtually unbeatable, though, over the last couple of months, winning 23 matches in a row in a year in which he has clinched the Australian Open, Wimbledon and US Open titles, while losing in the final of the French Open.

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This is as dominant as they get for any player in the modern era, but Federer, who beat Tomas Berdych 6-4, 6-2 believes the Serbian can be beaten and another title for the best player in the world is far from a foregone conclusion.

Federer will play Djokovic, who blasted past Nishikori 6-1, 6-1, on Tuesday in what look like being the match which will decide the winner of the Stan Smith Group in this ATP World Tour Finals in London.

"I don't think I'm that far off [from Djokovic's current level]," Federer, who knows a thing or three about dominating the sport, said. "That depends on who looks at the gap.

"Two days is not the match that I care the most about, to be quite honest. It's an important match, very important. But it's not THE match.

"This doesn't feel now so crazy early because we've had the week of preparation, we've both had a match now. I think we both feel like we're in the tournament.

"But nevertheless, it is only sort of a second round. Maybe still some open questions in both our games, or in mine, getting rid of what happened in the beginning of the match. It's going to be important not to have that against Novak clearly in a couple of days."

Federer said he was not happy with his start against Berdych, with the Swiss struggling to come to terms with the conditions at the O2 Arena, before, like he always does, finding his range to topple his opponent.

"It was one of the worst starts I've had in years," the world number two said. "No first serves, two double-faults, nothing coming from the baseline. So it was rough.

"But thankfully I was able to pick it up ... It's hard to blast through an opponent here in London, which I think helps for me now against Berdych in this case because I can make the match athletic.

"I was a bit sluggish coming in. I don't want to say too overconfident, but I thought it was going to be easier than that. It was a good lesson for me to learn."

Meanwhile, in the men's doubles, India's Rohan Bopanna and his Romanian partner Florin Mergea eased past the number one seeds Bob and Mike Bryan 6-4, 6-3 to open their campaign with an impressive win.