Christophe Fauviau, 46, from Dax, is accused of spiking the water bottles of 27 of his children's tennis opponents between 2000 and 2003 with the drug Temesta, an anti-anxiety medication which can cause drowsiness, in order to ensure victory for his son Maxime, now 19, and daughter Valentine, now 16.
It was a tactic that went tragically wrong when one of his son's opponents, Alexandre Lagardere, a 25-year-old primary school teacher, was killed after he crashed the car he was driving home from an amateur match which he had lost to Maxime Fauviau. Mr Lagardere had withdrawn from the game following the first set, after complaining of sudden, intense fatigue and loss of balance. He then slept for two hours before driving home. Police believe he fell asleep at the wheel. A post-mortem examination showed traces of Temesta in his system equivalent to four to six tablets of the drug.
Police suspicions quickly focused on Fauviau after complaints by two other players who had competed against Maxime in preceding weeks. One had spotted the former military helicopter pilot tampering with his water bottle shortly before the semi-final of a local village tournament in which he had been due to face Maxime Fauviau. He decided not to drink the water but gave it to police, who found it tested positive for Temesta.
The following day, Maxime's opponent in the final fell ill and was kept in hospital for days.
Fauviau was arrested in August 2003. Police say he admitted spiking the drinks of Mr Lagardere and two other players.
As details of his arrest spread, dozens of competitors who had played Maxime and Valentine came forward claiming they had suffered similar symptoms after matches, including extreme tiredness and blurred vision.
Fauviau faces charges of unintentionally causing a death by administering noxious substances. If found guilty he faces up to 20 years in prison.
Prosecutors will tell the court in Mont-de-Marsan that Fauviau is suspected of doping 21 rivals of his daughter and six of his son's.
All the players, nine of whom were under 15 at the time of the alleged doping, complained of weak knees, dizziness, nausea or fainting during the games. Some were later taken to hospital.
Fauviau is reported to have devoted himself to making champions of his children, particularly Valentine, said to be a gifted player. Maxime is reported to be less talented and prosecutors believe Fauviau could not bear to see him lose, so decided to give him a helping hand.
Psychiatrists who examined Fauviau say he was obsessed by his children's success and suffered from paranoid delusions that Valentine was being held back by "rotten officialdom".
A verdict is expected on 10 March.