Rafael Nadal
Rafael Nadal suffered a shock exit in the fourth round of 2016 US Open on Monday Reuters

Rafael Nadal's US Open campaign ended during the early hours of Monday when the two-time champion suffered a shock fourth-round loss to 22-year-old Lucas Pouille. The Spaniard though maintained he has not lost heart but vowed to push harder for better results.

In a four-hour battle, Pouille clinched the early lead before the match went to the decider. The young Frenchman held his nerves in the tiebreaker and advanced to the quarterfinals with a 6-1, 2-6 ,6-4, 3-6, 7-6(6) win.

"I have the motivation to keep working. But I need something else, I need something more that was not there today. I going to keep working to try to find," Nadal was quoted as saying by US Open's official website.

After suffering a wrist injury earlier in 2016, the Spaniard was forced to pull out of his favourite tournament - French Open. Nadal made a last-minute decision to withdraw from 2016 Wimbledon as well but made himself available for national duty during the recently concluded Rio Olympics, in which he won a doubles Gold and lost the Bronze to Japan's Kei Nishikori.

Nadal revealed that his wrist was improving and he was hoping to get back to his best form soon.

"My wrist is improving a lot. I say that looks like the injury is at the end of the process. I am ready to keep playing. That's what my body is asking me now," he said.

"Took a long time, a lot of hours, a lot of spirit of sacrifice to be back where I was before the injury this year. Was tough. And now I need to come back there," Nadal added.

Nadal was leading 4-3 in the final set before Pouille staged a fightback to take the match. The 30-year-old said he was disappointed to lose out on the advantage and have a good event at the Flushing Meadows.

"Is true that I don't have a lot of matches on my shoulders for the last three, four months, but even like this, I lost an opportunity. That's the real thing, no? That's the truth. I lost an opportunity to have a very good event here. I am sad for that.

"But I fought. I can play better. I can play worst. The only thing that cannot be bad is the attitude, no? Today the attitude was great. That's it," Nadal said.

The 14-time Grand Slam winner has had a title drought in the last two years with his last major win coming in 2014 at the Roland Garros. Nadal said the Monday's loss was not about the lack of mental or physical toughness but credited Pouille for coming up with a good fight.

"Mental or physical doesn't matter. I am out of the tournament. Physical, for sure no. I fight till the end. Of course was not a physical thing. Was not a mental thing. Was in terms of sport, and in sport you lose or you win. The opponent wants to win the same like you," he said.

Nadal was all praise for his young opponent. He labelled Pouille as top-10 material. "He's a player that has all the shots. He is a potential top-10 and a good fight for the big things the next couple of years. If he's able to keep playing well, keep improving," he said.