* World number 53 Troicki handed 18-month ban by ITF
* Tribunal says Serbian refused to give blood sample
* Loses money and ranking points from Monte Carlo Masters (Adds detail, byline)
LONDON, July 25 (Reuters) - Serbia's former world number 12 Viktor Troicki has been banned for 18 months after refusing to provide a blood sample at this year's Monte Carlo Masters, the International Tennis Federation (ITF) said on Thursday.
The 27-year-old, a Davis Cup team mate of world number one Novak Djokovic, was found guilty of "refusing or failing without compelling justification to submit to sample collection."
Troicki, who is now 53rd in the world rankings, is one of the most high-profile players to break doping rules in tennis and the sanction comes as the sport toughens its regulations.
An ITF statement said Troicki was notified on April 15 that he had been selected to provide a urine sample and a blood sample while taking part in the Monte Carlo event.
"(He) provided a urine sample, but did not provide a blood sample. He asserted to an independent tribunal that he was assured by the Doping Control Officer (DCO) that it would be acceptable not to provide a sample on account of him feeling unwell that day," the ITF's statement read.
"However, the tribunal concluded that the DCO told Mr Troicki that she could not advise him as to whether his reason for not providing a blood sample was valid, and that no such assurances were given by her."
The ITF said Troicki's actions constituted "a failure and a refusal" to provide a blood sample, and that his explanation for not doing so was not "compelling".
Troicki, who the tribunal heard was under stress at the time of the violation, will be suspended until Jan. 24, 2015.
His prize money and ranking points from Monte Carlo will be forfeited, although an ITF spokesman said that in "refusal" cases subsequent prize money and points are not taken away.
The Belgrade-based Troicki, who in 2010 won the final rubber for Serbia in the Davis Cup final, has played 10 tournaments since losing in the first round to Finland's Jarkko Nieminen at Monte Carlo, reaching the fourth round of the French Open.
He was in action right up until his suspension was announced, losing to Tommy Robredo 6-0 6-4 at the Croatian International tournament in Umag on Thursday.
The ITF said in May it would introduce biological passports for players this year, in line with measures adopted in other sports such as cycling.
The new system, under which test results are collated over time to enable testers to track any changes which might indicate doping, would involve more blood tests being done every year.
The ITF website (www.itftennis.com) says it conducted only 21 out-of-competition blood tests in professional tennis in 2011 compared to 3,314 carried out by cycling's world governing UCI. (Reporting by Martyn Herman; Editing by Ken Ferris)