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Casey Stoner said a fading rear tyre prevented him from pushing harder in the Indianapolis Grand Prix.

The reigning world champion grabbed the lead at the start but then lost pace and ultimately finished a distant fourth in the red-flagged and rain-affected race.

"I'm never going to be satisfied with fourth place but it would have been foolish to push harder in those conditions and I made the right decision not to do so," Stoner said.

"I got a good start but I quickly lost confidence because the rear tyre was tearing up quickly and by the time they stopped the race I was struggling to hold my lap times."

Team boss Livio Suppo wasn't sure why Stoner had encountered tyre problems, but was relieved that he brought the bike home fourth despite his issues.

"It was a tough race for Casey but he showed grit and determination to bring home a decent result," Suppo said.

"It may be to do with the power delivery of our engine or the set-up of the bike but the rear tyre went off quicker than expected and Casey couldn't ride on the attack, as we're used to seeing. It's a shame it wasn't a dry race because in the warm-up we saw Casey was in good shape for those conditions."

With Valentino Rossi winning the race, Stoner is now 87 points behind his title rival in the standings with only four rounds remaining.