Rafael Nadal completed a stunning comeback at the Barclays World Tour Finals in London yesterday as he clinched the year-end No.1 ranking for the third time, yet attention was swiftly deflected from that achievement by events elsewhere.

The Serb, Viktor Troicki, won an appeal at the Court of Arbitration for Sport in Lausanne to have his 18-month ban commuted to 12 months, for failing to take a blood test at the Monte Carlo Masters in April.The 27-year-old claimed he was ill on the day and had been misled by the doctor in charge into thinking he would be allowed to take the test the next day. Yesterday, Troicki claimed he had been "treated like a criminal" and made a scapegoat.

Now the International Tennis Federation, which administers the anti-doping programme, says it may give players the chance to ask for extra clarification.

"We have proposed a change to the rules of the Tennis Anti-Doping Programme, under which a DCO [doping control officer] may offer a player the opportunity to call for a supervisor or referee during a test in order to confirm the player's responsibilities," the anti-doping chief Dr Stuart Miller said. "If approved, this will come into effect in 2014."

Troicki, who reached a career-high ranking of 12th in 2011, received plenty of support from his fellow players, most notably his compatriot, Novak Djokovic, who criticised the doctor involved and said he no longer had any trust in the programme.

In a strong statement after his first match here on Tuesday night, Djokovic said the doctor's "negligence and unprofessionalism" were to blame.

Nadal again expressed sympathy for Troicki, saying the doctor "probably made a mistake" but the Spaniard said the biggest error of all had been the Serb's. "At the end we have rules," he said. "I am very sorry for Viktor because I believe 100% in him. But he knows that he has to pass the doping. So it was a big mistake." Back on the court at London's O2, Nadal duly took his place in the semi-finals thanks to a 7-6, 7-6 win over Stanislas Wawrinka.

"All season, I said it is not my goal, but the real thing is that after all the success I had during the season . . . I felt I deserved to be there," said Nadal. "Today was one of the best things that I did in my career."

Tomas Berdych kept alive his chances of making the semi-finals with a 6-4, 6-4 win over David Ferrer.