Audi says the current FIA World Endurance Championship regulations played against them in China as the grip on its superiority in the series loosens.

Having mounted a comeback against its Toyota rivals with wins at Le Mans and the USA, Audi has struggled to produce a convincing challenge in the most recent Fuji and Shanghai races, the R18 e-tron Quattro out-performed by both the Toyota TS040 and the Porsche 919 Hybrid.

Indeed, Audi suggests the characteristics of the Chinese circuit, namely its long straights, and the current regulations which see its superior diesel fuel economy hampered by a smaller fuel tank, do not play to the car's strengths.

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"Although the results are a little better than those in qualifying, they're obviously not satisfactory," said Head of Audi Motorsport Dr Wolfgang Ullrich. "We knew that it would be difficult here. The yellow period at the beginning helped our competitors with gasoline engines.

"They were able to refuel early and, as a result, saved a stop at the end. In our case, due to the prescribed fuel tank capacity and the stopping intervals, this wasn't possible. At Shanghai, we simply didn't have a chance on the long straights."

Having established itself as the benchmark in endurance competition this millennium, Audi looks set to cede at least the drivers' title to Toyota's Anthony Davidson and Sebastien Buemi, after their fourth within in China moved them to within ten points of clinching the title with two rounds remaining.

Audi also trails Toyota by 29 points' in the more lucrative manufacturers' standings.