(CNN) -- Roger Federer kept his hopes alive at the ATP World Tour Finals Thursday with a straight sets win over Richard Gasquet then repeated his demand for more rigorous doping controls in tennis.
"I just feel like we're not getting maybe tested enough," said Federer after completing a 6-4 6-3 win over his French opponent.
"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.
"Also last year when I won Dubai, Rotterdam and Indian Wells, and the year before that, I didn't get tested in one of those three events that I won. For me, that's not okay.
"You show up and test a guy that's winning everything. That's sometimes what I struggle with.
"But overall I trust the system. I think they're all very professional," the 17-time grand slam champion told gathered reporters.
Drug testing has been a hot topic all week at the end of season showpiece in London, with World No.2 Novak Djokovic mounting an outspoken defense of his Serbian colleague Viktor Troicki and saying he had lost faith in the anti-doping regime.
By contrast, Federer said he had little sympathy for Troicki, who saw his original ban cut from 18 months to 12 months Tuesday.
Troicki failed to provide a blood sample after feeling unwell at the Monte Carlo Masters but claimed he had been given permission by the doping control doctor to come back the next day.
"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.
"Like the test the next day for me is not a test any more because of what could have happened overnight.
"I believe whatever they decided on. I think it's just very important to give the sample when you're requested to give it because there you are in front of them and there is no way to escape. That's where we just need to be extremely firm."
Djokovic later beat Juan Martin Del Potro 6-3 3-6 6-3 in another tight three set match at the O2 Arena to book his passage into the semifinals.
It was his 19th straight win since losing to Rafael Nadal in the final of the U.S. Open and kept him on course in his title defense in London.
Defeat for Del Potro also boosted Federer's hopes of reaching the last four in a tournament he has won six times.
He will now meet the Argentine in a winner takes all final Group B match Saturday.