ATP World Tour Finals
Venue: O2 Arena, London Dates: 21-28 November
Coverage: One match each day live on BBC TV, streamed online & available through iPlayer; text commentary online and on mobiles for every singles match via BBC Sport website; every singles match live on 5live sports extra.
Nadal next plays on Friday against Tomas Berdych
By David Ornstein at the O2 Arena
Rafael Nadal put himself in pole position to qualify for the knockout stage of the ATP World Tour Finals with a victory over Novak Djokovic.
The world number one prevailed 7-5 6-2 in one hour 51 minutes to go top of Group A on two wins, while Djokovic, whose performance against Nadal was hampered by an eye problem, is level with Tomas Berdych on one win after he beat Andy Roddick in the afternoon match.
Nadal broke serve in game five of the first set but Djokovic hit straight back before requiring treatment for a complaint related to his right contact lens.
The world number three looked far from comfortable as Nadal broke again for 6-5, served out the opener and then struck two decisive blows at the start of the second set to help him ease home.
"I really feel sick talking about it because it's just incredible that this happened to me. It never happened to me in my life," said Djokovic.
"I'm really annoyed something like this can really affect the match. It did because my right eye got irritated and from the 5-5 I could not play. I could not see a ball, especially on the return. It was just terrible.
"I don't want to find any excuses for my loss, but the fact of the matter is that I just could not play. I could not play. These things happen. It's out of my control. Unfortunately, it happened in a very important match, in a very important moment.
"I will try to see a doctor and see if there is something more serious going on and in two days hopefully I can be ready."
The afternoon match between Berdych and Roddick provided no shortage of entertainment, yet everyone knew the piece de resistance was still to come.
With each passing hour before the 2000 GMT start time, the sense of anticipation around the O2 Arena would swell - and once the players emerged from their locker rooms, the atmosphere was electric.
The pair were squaring off for the 23rd time, and although Nadal won their last meeting in September's US Open final and arrived with a 15-7 head-to-head advantage, Djokovic had taken seven of their 11 matches on hard courts, including a round-robin triumph 12 months ago.
Nadal has enjoyed yet another undeniably glittering year, but he missed five weeks with a shoulder injury prior to this tournament and was pushed all the way by Roddick in his opening match on Monday, while Djokovic sailed past Berdych.
And it was the Serb who got off to the better start, reeling off a breathtaking service hold to love in game one and then bludgeoning his way to a break point in game two.
Nadal scraped to safety, but his opponent replied with four more service points of the highest order to keep the Spaniard under pressure.
However, the nine-time Grand Slam champion was equal to the challenge and held serve before turning the tables on Djokovic with some venomous attacking play of his own in game five.
Two break points came and went before he got the better of Djokovic after a pulsating net exchange that brought the 17,500 sell-out crowd to its feet.
But Nadal required some brief attention from the trainer at the change of ends and it seemed to affect his rhythm, a flat game six from the Spaniard enabling Djokovic to break straight back.
The contest was ebbing and flowing beautifully, the fans were getting behind their players in fairly equal measure and a tie-break appeared on the cards.
But now it was Djokovic's turn to call for help - the contact lens in his right eye causing him discomfort - and he even dashed to the locker room in a bid to resolve the issue.
"I don't even know what happened," added Djokovic.
Playing with one eye is not enough, especially if you have Nadal across the net
"The doctor told me he didn't see anything in my eye. Later on I had two lenses, then three lenses. They kept on going around and around. It was just so bad.
"I was feeling great on the court. I had time to prepare for this tournament mentally and physically. I put all my effort into it and had the desire to win. Then you get on the court and this thing happens to your eye, the most important part of your body, coordination, sight, everything you can think of.
"I needed some time to make it right, but I didn't have time. Everybody was hurrying me up. Three minutes, 30 seconds, come to the court, play your match. I just don't have time for that.
"Playing with one eye is not enough, especially if you have Nadal across the net. So basically I was trying to hit the ball as hard as I could. If it went in, it went in."
"I realised I could not make it right and I could not give my 100% on the court, so it was just a routine win for him in the second set."
The 23-year-old continued to be troubled on his return, and Nadal seized his opportunity to pounce, breaking to 15 for a 6-5 lead before serving out the set and letting out a huge roar of "vamos".
An angry Djokovic received extended treatment from the trainer and doctor at the turnaround but their efforts had precious little effect as he was easily broken in his first two service games.
Nadal was sensing an early end to proceedings and rubbed salt into his opponent's wounds by putting a up a couple of lobs that Djokovic lost in the lights.
The 24-year-old lost only five points in his first three service games of the set and swiftly found himself 4-0 ahead.
Djokovic's ailment had somewhat undermined the occasion and, despite picking up a couple of service holds, he sent a forehand long on match point to hand Nadal the spoils.
"It was a very good match for me," said the Spaniard, who failed to win a single set at last year's event at the O2.
"I'm very sorry for Novak. I wish him all the best for his eyes. This was my best match in London. The first 10 games were very level but I'm just happy with the two victories.
"It was an important win because my level was high another time. I played very well in the first set especially. And for sure it is a very important victory for the group because I am in a good position."