Things can get complicated during the ATP Tour Finals round robin, but back in 2009 things descended into a farce.The season-ending championship uses a unique format that is not seen on the ATP calendar for the rest of the year.Del Potro, and the crowd, were left in the dark about who had qualifiedGettySingles and doubles players are separated into two groups of four, within which each player plays three round-robin matches.The top two performers then go through to the knockout semi-final stage.However, it has faced some criticism that it can often lead to dead rubber matches – where either players have already qualified or already been knocked out.What happened at the ATP Tour Finals in 2009?There was a bizarre situation in 2009, where the event was hosted at the O2 Arena for the first time, where players and spectators were left in the dark about who had qualified.Juan Martin del Potro and Roger Federer faced each other in the final Group B match.Andy Murray had missed a chance to book his place in the semi-finals earlier in the day – winning against Fernando Verdasco but not in straight sets.Federer knew that a win would put him through, while Del Potro needed a straight sets victory to be assured of a position in the final four.The Argentine defeated the 20-time Grand Slam winner 6-3, 6-7, 6-3 and it created a whole lot of confusion.There was a three-way tie between Federer, Murray and Del Potro with all of them at 2-1 and level on sets.A lengthy period followed where officials took nearly 30 minutes to confirm the maths and work out who had progressed.Federer lost but still progressed to the ATP Tour semi-finalsGettyIn the meantime, Federer had left the court leaving Del Potro waiting with the crowd.To keep them amused, he brought out compatriot Carlos Tevez, who was playing for Man City at the time, for an impromptu knock about.Tevez’s tennis skills failed to impress so he resorted to doing keepy ups with the ball.Eventually organisers ruled that Federer and Del Potro would advance over Murray, effectively by a single game.“It was very strange but I don’t want to talk about that because the people who made the numbers to qualify, they know what they did,” he said. “Now it’s the past. I’m in the semi-finals, I’m happy for that, but it was strange.”Federer himself did not realise he had left Del Potro stranded on the court.Murray was the unlucky one to miss out on a semi-final berthGetty“Obviously the focus is completely on my own qualification,” said Federer. “The only thing that was important to me was the set I needed to win, trying to win the match.“The rest you block out and you concentrate on the match because you don’t want to lose against fellow rivals.“It’s not something I enjoy, to be quite honest. A lot of money, a lot of points on the line, so it would have been nice to go through with a clean sheet to the semis. I missed that, but it was against a good player today.”Federer added: “I asked Juan Martin myself at the net, ‘Did you make it or not?’ He said, ‘I don’t think so.’ So that’s the story I got.“Of course you’ve got to feel sorry for the guy who didn’t make it. At the same time, Del Potro beat the number one player in the world in the group, and I guess he also deserves to go through.“There’s only two places and that’s the way it is. It happens very, very rarely.”Tevez’s skills with a tennis racket did not match up to his skills with a footballGettyBBC Sport’s Oliver Brett said at the time: “What a shambles from the ATP at the end as they kept Del Potro waiting before finally revealing that he had gone through.”Del Potro went on to lose the final that year to Nikolay Davydenko, with the Russian beating Federer in the semi-final.