Defending champion Novak Djokovic booked his place in the semi-finals of the ATP World Tour Finals with a 6-3 3-6 6-3 victory over Juan Martin del Potro on Thursday.

After Roger Federer defeated Richard Gasquet earlier in the day, Djokovic knew any win against the Argentine fourth seed would guarantee his qualification from Group B with one match still to play.

And the world No.2 seized his chance, surviving a tough test lasting nearly two hours to extend his current winning run to 19 matches.

Del Potro can still make the last four as well, but he will have to beat Federer in their match on Saturday.

“With his serve it’s definitely a big advantage if I can get a break and then close it out,” Djokovic said.

“Thankfully I managed to do that.”

Four months ago, Djokovic out-lasted del Potro in an epic encounter that took nearly five hours and finished as the longest semi-final ever played at Wimbledon.

This was their first meeting in London since then and Del Potro took the attack to Djokovic early, securing two break points.

But, as was so often the case when they met at Wimbledon, Djokovic was able to get out of trouble with peerless ability to turn defence into attack in one motion.

The Australian Open champion had to stave off two break points early in the final set, but he gradually turned the tide and landed the decisive break for a 4-2 lead before serving out the win.

Meanwhile, Federer called for more drug tests in tennis beating Richard Gasquet 6-4 6-3 to keep alive his hopes of reaching the semi-finals of the World Tour Finals.

Federer said players are not tested enough and that he had no sympathy for the banned Viktor Troicki.

“I feel there needs to be more testing done. I feel like I used to get tested more, I think I was tested 25 times in 2003, 2004. Ever since, I think it’s been clearly going down this season,” said the six-time year-end champion.

Federer spoke after outrage from world No.2 Novak Djokovic over the ban handed to his fellow Serb Troicki for skipping a blood test after feeling unwell and coming back to do it the next day in Monte Carlo.

It was a transgression which originally cost him an 18-month suspension before it was cut to 12 months earlier this week.

“I do believe that when you are requested for a sample, you have to give the sample. It doesn’t matter how bad you feel. I’m sorry,” Federer said.