• ATP World Tour Finals

Djokovic hangs tough for semi-final spot

Novak Djokovic extended his winning streak to 19 matches © Getty Images

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Novak Djokovic is the second player through to the last four of the ATP World Tour Finals after the reigning champion saw off Juan Martin del Potro in a spectacular encounter.

The Serb joins year-ending world No. 1 Rafael Nadal in the semi-finals, and has now extended his winning streak to 19 matches after going the distance against Del Potro at the O2 Arena to claim a 6-3 3-6 6-3 victory.

The Verdict

• With Novak Djokovic coming through the group unscathed so far, it is now a direct shootout between Juan Martin del Potro and Roger Federer for a semi-final spot.
• Only once has Federer fallen at the round robin phase of the event, which was back in 2008, and the Swiss has been to the last four or better on 10 separate occasions.
• Del Potro almost grabbed the title in 2009 the first time the event came to the O2, and it is set to be a fascinating contest between two players who have met twice in recent weeks.
• Del Potro upset both the crowd and Federer at the Swiss' home event in Basel to retain his title there, while Federer avenged the loss with a win over Del Potro in the quarter-finals of the Paris Masters last week. It all ensures for a thrilling match on Saturday.

The previous two meetings between Djokovic and Del Potro were two of the greatest matches of the season. Djokovic won both encounters after edging out the Argentinian in a five-hour epic in the semi-finals of Wimbledon, and won a deciding set tiebreak to clinch the Shanghai Masters in October.

And once again the pair put on a thrilling spectacle at the World Tour Finals, with Djokovic getting the better of Del Potro for the fourth time in 2013.

Djokovic was forced to save two break points in the opening game of the match as Del Potro signalled his intentions to attack the Serb. However, in the fourth game, the roles were reversed as Djokovic put the pressure on Del Potro with three opportunities to break.

Neither were converted, but when Djokovic moved into a 3-2 lead, it was the reigning champion who pulled further clear by capitalising on his second break point en route to carving out a healthy 5-2 cushion.

Del Potro closed the gap to two at 5-3 with a lightning-quick service game, but could not deny Djokovic from wrapping up the first set.

After Del Potro went 3-2 in front in the second set, he was aided by a fortunate bounce from the net cord which left Djokovic rooted to the spot - a shot that not even the Serb could chase down - and after 35 minutes the former US Open champion levelled the contest.

Fending off two break points in the third game of the deciding set proved to be crucial for Djokovic, as he converted his with some fine returning to break for 4-2.

And like he did at Wimbledon and in Shanghai, Djokovic ended the brave resistance of his opponent to clinically close out the match with a backhand down the line to maintain his perfect record since losing the US Open final.

Having now qualified with a match to spare, Djokovic was delighted to be through.

"Considering the schedule of back-to-back at Bercy [Paris Masters on Sunday] and the World Tour Finals, I had a bit of luck that my group was selected to play on Tuesday and Thursday, and I had night matches, so it gave me more time to rest and get ready," Djokovic said.

"I overcame two big fights against Roger [Federer] and Juan Martin as both matches went the distance, so I'm very glad to be able to play my best when it's needed and qualify for the semi-finals."

Djokovic won all five of his matches 12 months ago en route to the title, and the Serb will not be easing off despite sealing a last-four spot.

"To qualify is a positive, but I still want to try and win the last match of the group [against Richard Gasquet]," Djokovic stated.

Del Potro was left to rue the errors that handed Djokovic the decisive break in the final set.

"I missed three forehands in a row, and I made easy mistakes, ones that I never expect to make," he said. "I think I played a good match, but the mistakes were the difference tonight."

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