The International Tennis Federation (ITF) is confident its anti-doping programmes are working effectively.
Andy Murray has called Victor Troicki and Marin Cilic "unprofessional", adding to the debate about tennis' anti-doping rules.
Viktor Troicki said his dream of being a "top player" had been taken away after his doping ban was reduced to 12 months from 18.
Stanislas Wawrinka beat Feliciano Lopez 6-3 3-6 6-3 at the Paris Masters to end Nicolas Almagro's World Tour Finals chances.
Roger Federer confirmed his spot at the end of season ATP Tour finals by reaching the third round of the Paris Masters.
Marin Cilic won his first match since returning from a doping ban with a 5-7 6-1 6-4 success against Igor Sijsling in Paris.
Marin Cilic has called his appearance at next week's Paris Masters "the gift of the year" after serving a doping ban.
Tennis player Marin Cilic has had his nine-month ban for a doping offence reduced to four months by CAS.
Marin Cilic cited a knee injury at Wimbledon this year to hide the fact he had failed a dope test, according to ITF.
Former world number nine Marin Cilic has said he will be appealing a nine-month doping ban imposed on him.
Marin Cilic has been banned for nine months after testing positive for a banned stimulant.
Great Britain won promotion back to the Davis Cup World Group as Andy Murray beat Croatian's Ivan Dodig 6-4 6-2 6-4.
ATP World Tour Finals
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