Novak Djokovic
Djokovic was at his resilient best as he stormed into the last four in London. Getty Images

Five-time champion Novak Djokovic became the first player to reach the semi-finals of the ATP World Tour Finals after an extraordinary defeat of Milos Raonic in straight sets at The O2 Arena. Djokovic prevailed via tie-breaks in the first and second sets to win 7-6 7-6 and march into the knock-out phase as the winner of the Ivan Lendl group as he continues to hunt down world number one Andy Murray.

The Serbian took a tense first set after saving three break points in the opening two games, benefitting from a double fault from Raonic in the tiebreaker. Djokovic was twice a break up in the second set but Raonic stormed back into the contest, but at set point missed the chance to force a decider.

Djokovic rallied and showed the mental fortitude to take the second set with a superb back-hand half volley at the net, before Raonic skewed wide a backhand of his own to bring the curtain down on the most remarkable of matches which lasted two hours and 14 minutes. The result means Raonic faces a straight shoot-out with Dominic Thiem to join Djokovic in the semi-finals, while Frenchman Gael Monfils is out having lost his opening two round robin matches.

"I guess I had to hang in there mentally and believe that a moment will occur," he said upon qualifying for the last four. "You have to take them. Winning the first set gave me some wings and I was twice a break up in the second. I think a couple of points separated us today and it could have gone either way,

"He is very strong physically so his serve is weapon. He's got some firepower especially not the forehand side and the backhand corner and play inside out. His game plan was to be aggressive so I was doing a lot of running."

Victory for Thiem earlier in the day had opened the door for Djokovic to book his place in the last four, a victory which would also dump Monfils out of his debut World Tour Finals. But regardless of the qualification scenarios in play, the 12-time major champion's pursuit of world number one Murray dictated that he could ill-afford any slip-ups this week.

Milos Raonic
Raonic missed countless opportunities to put pressure on Djokovic. Getty Images

Raonic could only book his own place in the semi-finals if he prevailed without dropping a set but he too had one eye on the ATP rankings as he looked to leapfrog Stanislas Wawrinka into number three in the world. Having only taken one set off Djokovic across the pair's seven meetings, Raonic was also eager to use the round robin clash to prove he could mix it with the world's best.

The 25-year-old showed no fear from the outset and Djokovic needed an ace to see off a break point chance in the opening game. Raonic had two further opportunities to go ahead in the next service game, but went long on the forehand side as Djokovic served his way out of trouble.

Those chances appeared increasingly distant as Djokovic belatedly discovered some consistency on serve, as Raonic's forehand began to be tamed. As the first set went to a tie-beak, the Canadian grabbed a mini-break, but a miss-cued backhand and long forehand helped Djokovic grab the initiative.

A virtuoso forehand from Raonic prevented him going three set points down, but a miscued overhead volley gave Djokovic the chance to close out the set. The Serbian went long in a desperate attempt to keep Raonic at bay, but a double fault after the changeover handed the crucial first set the way of the world number two.

Another bout of nerves on serve in the form of a second double fault gave Djokovic a routine break at the beginning of the second set as the defending champion recaptured the resilience which had defined his 122-week run at the top of the ATP standings.

Two superb winners down the line saw Raonic break back but he was unable to consolidate as he netted two backhands from the very next service game. Within six points of victory Djokovic couldn't convert the opening and as he dumped a forehand into the net and simultaneously reignited Raonic's challenge.

Raonic missed the opportunity to take the match into a third set as Djokovic served at 5-6 down, thundering a backhand into the highest part of the net and allowing the 2010 Davis Cup winner off the hook. At the changeover in the subsequent tie-break the players couldn't be separated, with Raonic again relinquishing a mini-break.

Another engrossing rally saw Djokovic's razor-sharp reactions prove too much for Raonic, before an ace brought up a first match point. Raonic found the tramlines via a wayward backhand to confirm the victory for Djokovic, who despite not being at his flowing best produced a display which proves he is ready for the challenge of hunting down Murray.