• ATP World Tour Finals

Federer and Del Potro go head-to-head to reach last four

Roger Federer and Juan Martin del Potro are fighting for the last semi-final spot © Getty Images

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While there were no fewer than seven qualification scenarios for Group A of the ATP World Tour Finals, the formula is very simple for Roger Federer and Juan Martin del Potro in Group B at the O2 this afternoon.

No need to draw out the calculators and work tirelessly on the different permutations for the toughest group of the competition. Win, and you are in the last four of the competition. Lose, and your season has come to a somewhat disappointing end - especially if you are the once all-conquering Federer.

Novak Djokovic put himself in the luxurious position of making it through to the semi-finals with a game to spare after beating the pair in his first two matches, and you could feel the relief of the defending champion in his press conference - a stark contrast to when he lashed out over the anti-doping ban of Viktor Troicki earlier in the week - after requiring to dig deep to see off Del Potro late on Thursday night.

Who does the Serb think will join him out of the group? "I can expect a three-set battle, but I can't pick the winner," Djokovic replied with a shrug of the shoulders.

Djokovic at the start of the tournament believed Federer's slump in 2013 has been down to his movement slowing down. Now, two games into the competition, what are his thoughts on the legendary Swiss?

"Roger is definitely playing better than he was throughout the whole year. He loves these conditions and he likes this court." Compliments from Djokovic, which are always easy to hand out when you defeat the person you are offering them to.

Encounters between Federer and Del Potro have thrown up some spectacular battles, and kept audiences on the edge of their seats. The US Open final of 2009, the London 2012 Olympics and the French Open of the same year stand out, with the US Open being the scene of Del Potro's one and only grand slam triumph and the most painful defeat of Federer's career.

With Federer's place at the World Tour Finals not secured until the last event, the Paris Masters, the Swiss is looking to the tournament he has won a record six times to resurrect his miserable campaign.

"I'm happy to be part of that funny game again," Federer joked when questioned on the possible scenarios in his group if Del Potro had beaten Djokovic - which could have been decided on rankings, meaning Federer would have been out if all players finished level in the group.

Now, luckily for the 17-time major winner, rankings will not come into play, and there are no difficulties in solving who will go through to the semi-finals. Del Potro had racked up three consecutive wins against Federer before the latter trumped him in Paris last week, and that pleased the Swiss.

"The win in Paris was big for me. I'm glad I got one back in Paris because I knew we could be in the same group playing each other," he said, revealing a change in himself after the year he has had. "I truly believe my confidence is higher, and that's what I need to beat the best, and Juan Martin is part of that group."

Federer can also draw confidence on the fact that only once has he failed to reach the semi-finals in 11 appearances at the World Tour Finals.

Del Potro became the first man to lift four ATP 500 titles in a year when he silenced the crowd to beat Federer in his home town in Basel, and the Tower of Tandil has been running on empty ever since he arrived to London.

The Argentinian dragged himself to spend some time with tournament sponsors Barclays and their ball kids earlier in the week at the O2, and surrounded by the group of 30 he said they "make our job very easy". He probably wishes the same could be said of the tussle with Federer.

Del Potro had a bumpy ride into London after his bag was stolen while signing an autograph before waiting to board a train from Paris, and the loss of his cherished rosary which was blessed by Pope Francis earlier in the year has weighed heavily on his mind.

As this is the final event of the year Del Potro is one of the players who could do with recharging his batteries. Whereas Federer has a spring in his step after only winning one title in 2013 and exiting tournaments early, Del Potro appears to have hit his limit.

"I will try to do my best. I know for me it has been a long season and a lot of matches towards the end of the year, and I'm getting tired," he summoned the energy to say after seeing Djokovic depart the conference room before him with a beaming smile. "But it is the last effort, and it could be the last effort of the year, so I will try to enjoy it and do the best I can."

Even if he would not go as far as predicting a winner, we will leave Djokovic to have the concluding say on the semi-final showdown, given he is responsible for Federer and Del Potro being involved in Saturday's shootout.

"All the matches that they have played against each other this year went the distance."

That is as close to a prediction as we are going to get, and it is hard to disagree with the reigning champion.

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