Fabio Fognini has admitted that he struggled with fatigue in his straight-sets defeat to Feliciano Lopez in the fourth round of the US Open.

The Italian produced a shock on Friday by coming from two sets down to record a memorable win over Rafael Nadal, but he concedes that his efforts in that match affected him against Lopez, who will meet Novak Djokovic in the next round.

"I mean, if I have to tell you something, for sure today I was much tired than yesterday. Yesterday I was feeling good. I would say," Fognini told the US Open website.

"But tonight and this morning I woke up really, really - not bad - but bad sensations. I was feeling my shoulder, my knee. Everything almost, but I think it is normal.

"I'm happy to play the fourth round. I have to be happy, of course, because I won three good matches. Hopefully I am going to repeat this one in the future."

Fognini had previously gone six Grand Slams without reaching a fourth round.