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Roger Federer has called for tennis to get tougher on doping - but Novak Djokovic believes the rules are already too strict.
The pair are in London ahead of the ATP World Tour Finals at the O2 Arena, where they have been drawn in the same group and will meet for the 43rd time next week.
With the sporting week dominated by the scandal in Russian athletics, tennis' anti-doping programme again came under the spotlight - and drew differing responses from world number one Djokovic and 17-time grand slam champion Federer.
Federer said: "I think it's very important. The player needs to feel that there is going to be tests often to shy them away from the stupid thoughts they might be having."
Tennis' anti-doping programme falls under the jurisdiction of the International Tennis Federation and the exact amount spent on it is not made public.
But what is clear is it is a fraction of the prize money paid out at the grand slam tournaments.
Federer believes there is no excuse for not testing players after every match once they reach the quarter-finals of tournaments.
He said: "I think they try their best but I think we could always do more. We have a very clear thing of what we should be doing - more testing.
"In my opinion where the points become greater, the money becomes greater and we need to be tested. It is very simple. That's how you scare off people.
"I don't understand that sometimes you have a run and you win a couple of events, and the next thing you know you haven't been tested. It just can't be that way.
"I'm always surprised when I win a tournament, I walk off the court and it's like, 'Where's the doping guy?' I don't get that. I hope in the future it's going to be better."
Meanwhile, Federer added of the Russia scandal: " I think there were reports of nations not obeying the rules, so I'm not surprised."
Djokovic launched a stinging attack on the anti-doping programme at the World Tour Finals in 2013 after his good friend Viktor Troicki was given a 12-month ban for missing a doping test, claiming he had lost trust in the system.
On Friday he backed the programme but criticised the 'whereabouts' policy for being too strict. Athletes have to inform the testers of where they will be for a period each day to enable random out-of-competition testing.
Djokovic said: "The whereabouts demands are a little bit too much and a bit unnecessary to write where you are every single day of the year.
"If you don't appear at the place where it is written down then you get a warning, then two warnings and then suspension. I think that is a bit too much.
"The tennis season is very long. They know where we are and they can find us. When you're in the off-season you are going back and forth and changing cities and locations and it can be hard to track down and fill in the whereabouts sheet."
Rafael Nadal has also been a critic of the anti-doping programme and he reacted with a certain exasperation at again being quizzed about it.
The 14-time grand slam champion believes the issue would be put to bed if the number of tests each player had to take was made public.
He said: "We have a healthy anti-doping programme that works well. The only thing in my opinion that will be great is if one day the controlled anti-doping that we do is 100 per cent public.
"I don't know if I had 10 or 12 or 20 (tests) this year. If one day the ITF decides to make all these controls public, it will finish these questions and, the most important thing, it will finish the thoughts about if it's enough or not enough."
The World Tour Finals begin on Sunday, with Djokovic taking on Kei Nishikori and Federer playing Tomas Berdych.
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