Rafael Nadal admitted he does not currently deserve to be number one in the world.

This was after losing to Robin Soderling in his first group match at the Barclays ATP World Tour Finals in London on Monday.

The Spaniard went into the end-of-season event knowing he had an outside chance of overhauling Roger Federer for the year-end top spot but that surely went with his 6-4 6-4 defeat at the O2 Arena.

"I think the chances I have are not here, not to be number one," he said.

"I think it is not the right moment. My level right now I think is not to be number one.

"Sport is like this. Sometimes you have best moments, sometimes you have little bit worse moments. This probably is one of these moments for me right now."

Just like Federer yesterday against Fernando Verdasco, Nadal dropped his opening service game - although, unlike the world number one, he recovered the break quickly.

But that did not shake Soderling's belief and a netted Nadal forehand gave the 25-year-old a set point on his opponent's serve, which he took thanks to an uncharacteristic unforced error from the Australian Open champion.

In the second set it was the Spaniard who broke first but Soderling simply upped the power level and forced three break-back points - taking the final one courtesy of a successful Hawk-Eye challenge.

Although there were flashes of Nadal brilliance - notably some stunning passing shots - they were too infrequent to really put the underdog in trouble.

And, in a repeat of the opening set, the increasingly-frustrated Majorcan found himself staring down the barrel in the 10th game - but this time a break point for Soderling was match point.

Nadal saved the first one but the second chance was gleefully gobbled up by the Swede courtesy of a wayward Nadal backhand.

It was Soderling's second high-profile victory over the six-time grand slam champion this year after he ended Nadal's unbeaten record at the French Open in one of the shocks of the season.

Nadal felt his failure to produce his best on the big points was ultimately what cost him the match.

He continued: "I didn't play really bad. But in the important moments I didn't have this necessary calm to win these matches against the best players.

"And this confidence, at that moment, that's what was wrong today."

Soderling, who only qualified for the eight-man tournament because of Andy Roddick's withdrawal through injury, admitted he had expected to be pushed harder on the big points at the end of each set.

The Swede said: "If you look at the top guys, they play well on the important points. He made two, I wouldn't say unforced errors, but he missed two makeable points."

Soderling was pleased with his aggressive tactics, adding: "I think I was hitting the ball well today. And I think that's how I have to play against Rafa to have a chance to beat him.

"It's tough to keep on playing these long rallies every time because he's a great fighter and he's moving really well. So I tried to keep the points short and it worked today."

The night match was an epic affair, with defending champion Novak Djokovic eventually battling past Nikolay Davydenko 3-6 6-4 7-5 as the clock approached midnight.

It was a repeat of last year's final, which the Serbian won easily, but not so today as he found his Russian opponent in top form.

Djokovic finally broke Davydenko at the end of the second set to level proceedings and then served out the victory at the second time of asking - the world number seven breaking back for 5-5 with a stunning backhand only to throw it all away with a poor service game.

Djokovic admitted he was struggling physically but was delighted to come through.

"Generally I'm very exhausted because of the long season," he said. "He was a better player in the first two sets and maybe overall in the whole match.

"But when you have bad days, you still find a way to win. I think that's something that is great to have in your game."

Davydenko called for the doctor in the second set and revealed afterwards he was again troubled by breathing problems that have dogged him throughout the indoor season.

He said: "Some doctors say maybe asthma, some say maybe allergy, some say bronchitis. I really don't know. If you see all the matches that I lose in three sets, that feeling happened like this."

He continued: "I know I showed my good tennis today. It's really great, but I lost."