* Djokovic proves tough nut to crack
* Serb beats Del Potro to reach semi-finals
* Federer still in hunt for semi-final place
The hardest nut in men's tennis again proved resistant to the heaviest of sledgehammers as Novak Djokovic absorbed the brute force of Juan Martin del Potro to book his place in the last four of the ATP World Tour Finals on Thursday.
Djokovic's elastic limbs propelled the Serb to a 6-3 3-6 6-3 Group B victory that leaves Del Potro to fight it out with Roger Federer for runner-up spot.
Federer, beaten by defending champion Djokovic in straight sets on Tuesday, got back on track with a morale-boosting 6-4 6-3 victory over Frenchman Richard Gasquet who can no longer reach the semi-finals even if he beats Djokovic on Saturday.
The six-times former champion lost the opening six points and needed six match points to finish off Gasquet but looked more like his old majestic self as he produced some champagne moments for his legion of fans - one topspin lob to avert a break in the second set the shot of the day.
"I guess those are the kind of matches I need right now. You know, straight sets against a good player. It clearly came at the right time right now," Federer, who has only one title this year but scooped the Arthur Ashe Humanitarian of the Year award, as well as the ATP fans' favourite and Stefan Edberg Sportsmanship awards this week, told reporters.
"There was pressure after losing my first one. It was a big win for me and I hope now to keep it up against Juan Martin."
He had better strap on some heavy armour when he meets Del Potro on Saturday because the Argentine world number five was striking forehands and serves with fearsome force against Djokovic, even if he did blow himself out in the end.
It would have overwhelmed almost any other player in the world but only rarely did he manage to blast through Djokovic's defences as he took the second set of a high-octane duel.
The last time Djokovic and Del Potro locked horns in London back in July they produced the longest Wimbledon semi-final ever - a near five-hour match of relentless baseline savagery.
They were at it again at the o2 on Thursday and although Djokovic won for the 11th time in their 14 encounters there was plenty of evidence to suggest Del Potro will be pushing the Serb and world number one Rafael Nadal for the majors in 2014.
For the moment, however, it is all about Djokovic, who has won 19 consecutive matches since the U.S. Open, and Nadal who got the better of him that day in New York.
Thursday's match was the 78th of Djokovic's season but kitted out like an assassin in all black he bristled with energy, oozed confidence and was clinical on the big points.
It is almost as if he does not want the season to end.
"I had a great start to the season and a great ending," the Australian Open champions told reporters. "I would pick these two parts of the year as the best for me.
"I just feel well at this particular part of the year. I know most of the players are exhausted physically, mentally, emotionally, after a long season.
"But, you know, I try to find that as an opportunity for me to get extra motivated to kind of push myself to the limit and deliver the goods when needed."
Del Potro, still only 25, has also been on a hot streak since the U.S. Open, winning titles in Tokyo and in Basel where he beat Federer in his own backyard, and after beating Gasquet on Monday, looked in the mood from the start against Djokovic.
However, he paid for failing to convert break points in the opening game and Djokovic struck first blood in the sixth game with a succession of laser-guided groundstrokes that eventually proved beyond even the condor-like reach of Del Potro.
One break was sufficient to wrap up the first set but Del Potro was not finished and hammered away in the second, earning a decisive break in the sixth game with a netcord.
Del Potro piled on the pressure at the start of the third as he cranked up the power on his forehand wing.
Two break points arrived at 1-1 but once Djokovic had wriggled out of danger he did not look back.
Having beaten Federer and Del Potro so far here, the Serb was asked who he thought would win their showdown.
It was about his only moment of doubt all night.
"Roger is definitely playing better than he was throughout the whole year," he said. "He loves these conditions.
"But on the other hand, Del Potro has big serve. I can expect a three-set battle. I can't pick the winner."