He had proved most disappointing in four runs since winning the Lingfield Derby Trial by seven lengths in May, but clearly relished yesterday's near two-mile trip in soft ground.
Sent to the front by Richard Quinn a furlong-and-a-half out, he stayed on strongly to beat Henry Candy's Always Friendly by two-and-a-half lengths, with odds-on favourite Sought Out six lengths away in third.
'He loves this mud,' confirmed Quinn. 'He's a nice horse, but he's very lazy and I had to work at him all the time.
'The last time he ran in France he hit the front too early and idled. Today I waited until the last quarter-mile before we made a move.'
Keen Hunter, runner-up in the Prix de l'Abbaye earlier this month, seemed to have been found an easy opprtunity to get off the mark for the season in the Group Three Prix du Petit-Covert, but was beaten three-quarters-of-a- length by the juvenile Wixon.
'I was very confident before the race,' said Wixon's trainer, Francois Boutin. 'I thought about running her in the Abbaye, but she wasn't quite right for that so I ran her in the Prix Eclipse where she finished second'.
(1m 7f 110yds Group One Prix Royal Oak)
1. ASSESSOR T Quinn
2. Always Friendly A Munro
3. Sought Out C Asmussen
12 ran. 2 1/2 , 6. (trained by R Hannon). Pari-mutuel (to a 1 franc stake): 43.70; 4.70, 2.90, 1.30. DF: 153.70.