Rafael Nadal insists he is still capable of challenging for grand slams despite his surprise fourth-round loss to Lucas Pouille at the US Open.

The Spanish fourth seed was stunned in a 6-1 2-6 6-4 3-6 7-6 (8-6) defeat to Frenchman Pouille at Arthur Ashe Stadium on Sunday.

For the first time since 2004, Nadal failed to reach the quarter-finals of a grand slam in a calendar year – with his run at New York his best.

But the 30-year-old believes he knows what he needs to work on, saying: "You can see it or you can write the way that you want. I know what's going on. I know what I have to do.

"When you are able to play semi-finals of Masters 1000s, winning tournaments like Barcelona, playing good tournaments in the other events, it's not a question of level to be in the final rounds of the grand slams.

"It's just a couple of things that for some reasons didn't happen, and you need to be ready to have the auto-critic. You need to be auto-critic.

"It's something that I believe is really going to change."

While Nadal paid tribute to Pouille, the 14-time grand-slam champion did have his chances – leading 4-2 in the fifth set.

He also sent a short forehand into the net at 6-6 in the tie-break, but denied pressure was the reason.

"After winning 14 [grand slams] and being in semi-finals a lot of times, you feel that's pressure?" Nadal asked.

"I'm 30 years old, after having the career that I have had, it's not a question of pressure."