Aleix Espargaro overtook seven riders in the space of eight laps in Sunday's COTA race, only to suffer his fifth and final fall of a punishing MotoGP weekend.

The lone Aprilia rider present after the withdraw of Maverick Vinales, the bumpy Texan circuit proved the toughest challenge of the season so far for the improved RS-GP.

Starting just 19th on the grid, Espargaro was up to twelfth when he lost control of his machine on lap 9 of 20.

Most frustratingly, the Spaniard insisted he wasn't pushing at the time of his fall, losing well over one second a lap to the leaders, and doesn’t understand why the accidents happened.

"It has been a nightmare, sincerely. My frustration level is very high because actually I don’t have a clear response to [why we had so many accidents]," Espargaro said. "Obviously the bumps didn’t help me. My bike was very difficult to ride. But the bumps were there for everybody.

"Until now, we have been competitive everywhere. In Misano I was P1 at the test, in other tracks we fought for eighth place, but more-or-less our level was similar. But at this track I didn’t find a way.

"I started to pass some people in the race - Morbidelli, Lecuona, Dovizioso, Alex Marquez - but my pace was not fast. I was not pushing so much. I was losing one-second a lap to the leaders or even more. But I crashed again. So I have no explanation.

"I am very sorry for my team because to have just one Aprilia in America and then crash five times and go home with no points is not good. I'm angry and frustrated but it is how it is and hopefully we can find an explanation.

"Because we can forget this weekend - and the season has been very good so far - but for me I need an explanation. I need to understand why. Right now I don’t know why."

Espargaro's fourth DNF of the season means he has dropped to eighth in the world championship, behind Sunday's winner Marc Marquez.