Rafa Nadal kept the one major title missing from his glittering CV firmly in his sights on Wednesday as he took brutal advantage of Novak Djokovic's eye problems to notch his second win at the ATP World Tour Finals.

The world number one and three were locked in a ferocious baseline battle for more than hour before Djokovic lost his focus, literally, and Nadal raced away to win 7-5, 6-2 in front of a full house at the 17,500-capacity O2 Arena.

"You know, playing with one eye is not enough, especially if you have Nadal across the net," a dejected Djokovic, who needed treatment for what he described as "irritation" in his right eye, told reporters.

"All I could do was basically try and hit the ball as hard as I can. If it goes in, it goes in," he added.

A true gentleman off the court, Nadal rarely shows any mercy on it, and after being matched by Djokovic for most of a 76-minute first set he seized on his opponent's misfortune to pocket the second in little more than half an hour.

Nadal tops Group A with two wins from two matches, although the complexities of the round robin format used at the Tour's season-ending showpiece mean the Spaniard is still not assured of his semi-final spot.

All four players in the group can still progress, including Tomas Berdych who rediscovered the form he showed in reaching the Wimbledon final in July when he notched his first win here, beating American Andy Roddick 7-5, 6-3.

Berdych and Djokovic both have a win each going into Friday's final group matches when Nadal faces Berdych and the winless Roddick takes on Djokovic, who said he will have his eye checked out by a doctor.

"I have big chances to qualify still. It's just that these things make me very angry," Djokovic said.

"From 5-5 I could not play. I could not see a ball, especially the return. It was just terrible," he added.

It was a shame for the evening crowd who had been royally entertained by two players at the peak of their powers.

After expressing his sympathy for Djokovic, however, Nadal stressed that he is taking nothing for granted.

"I've won two important matches of the round robin but I am not qualified," he said.

"I can meet with the island of Mallorca in the semi-finals or I could meet with Roger Federer or with everyone in the other group.

"We'll see what going to happen after tomorrow. It's not going to be easy against Tomas," he added.

Roddick's defeat, like Djokovic's problem, came out of nowhere. He looked in control when he had two set points in the opening set at 5-4 on the Berdych serve but the match swung quickly in favour of his Czech opponent who since Wimbledon had won just eight matches.

"There's always normally some sort of chance of something," Roddick said of his prospects of reaching the semis.

"This format gives us hope, even if it's false hope sometimes," he added.

World number two Roger Federer leads Group B with two victories from Robin Soderling and Andy Murray, who both have one each.

The top two places in that group will be decided on Thursday when Federer plays Soderling and Murray takes on Spain's David Ferrer.