Behkabad needed every yard of the mile and a half to get the better of Planteur in the Qatar Prix Niel at Longchamp.
The latter tracked his pacemaker Vivre Libre before taking over coming out of the false straight.
Behkabad, trained by Jean-Claude Rouget and ridden by Christophe Lemaire, was soon in close attendance but it was only in the shadow of the post he got up.
Winning owner the Aga Khan said: ‘He is a nice horse and he will improve for this.
‘I always like to go away and look at things again, but in principle both he and the filly (Sarafina, third in the Vermeille) will both run in the Arc, as at this stage of the season it is not a problem to run a filly against the colts.’
Duncan crowned a fabulous weekend for the Classic-winning pair of John Gosden and William Buick with victory in the Qatar Prix Foy.
Successful at Doncaster 24 hours earlier with Arctic Cosmos in the Ladbrokes St Leger, Buick again advertised his talents in the saddle as he produced a masterful front-running ride on the game five-year-old.
His mount was briefly headed in the straight by Timos but fought back gamely to take the Group Two prize from Japanese raider Nakayama Festa.
Gosden said: ‘The English horses always have to make the running over here, but he was very genuine and when the Japanese horse came to him he fought him off.
‘He has won a trial and deserves the chance to go to the Arc, but of course that will be a different ball game.’
Swiss Diva showed what an improved sprinter she has become by spreadeagling her field in the Qatar Prix Du Petit Couvert at Longchamp.
David Elsworth's four-year-old hit the front well over a furlong out after Bluster had taken the lead briefly, and quickly put daylight between herself and the opposition.
She had such a healthy lead that her rider Ioritz Mendizabal allowed himself the luxury of easing her down close home, scoring by two lengths and a head.
Bluster hung on for second place ahead of Mar Adentro.
Elsworth said: ‘The jockey seems to have a great rapport with her, I'd like him to keep the ride if he can.
‘She picked up really well and won nicely, so she will be coming back here in three weeks' time for the Abbaye.’