Ben Spies has revealed a faulty tyre was to blame for his lacklustre performance during the second World Superbike race of the day at Magny-Cours, his fourth place finish overshadowing a dramatic victory in the opening encounter.

Having come into the weekend three points adrift of Noriyuki Haga in the overall standings, Spies inched back ahead by two points following a thrilling first race in which he shrugged off a last lap mistake to beat his title rival.

However, a problem with the front tyre on his Yamaha WSB R1 would see Spies finish a lonely fourth in race two, the American being forced to watch winner Haga stretch his advantage to ten points with only one round and two races remaining.

Visibly frustrated at being unable to push in the second race, Spies remained philosophical about the situation, adding that he was merely lucky to reach the finish line.

"I tried as hard as I could in that second race, but just went backwards," the MotoGP bound rider said. "It's unfortunate, but we got some sort of faulty [front] tyre in the second race. It was a bad time for that to happen, but it's part of racing.

"It felt like I had a motorhome tyre on the thing and there was nothing I could do. With the way I had to ride I was fortunate to be able to finish in the top ten, so I feel very lucky to get fourth."

Prior to that, Spies had celebrated an important victory over Haga during race one having led from almost lights-to-flag. Still, he was made to work hard for victory after an error on the final lap allowed Haga to briefly take the lead, only for Spies to collect himself and regain the position before holding on to the finish line.

"I was making a bunch of little mistakes in the first race," Spies admitted. "I should have won that going away, but with the mistakes Nori was able to catch me. I ran wide in that corner and Nori got by me for about a half second.

"I knew I could get back under him, so I squared it up, got on the gas and got back in front. I figured he might make another attempt to get by, but I was able to hold him off."

Despite the setback, two wins for Spies at Portimao, one of the few circuits he has tested on before, would be enough for him to win the title, regardless of Haga's performance.