Rafael Nadal believes Andy Murray could be paying the price for his achievements last year, but still thinks the struggling world number one can challenge for a first French Open title.

Murray's poor season suffered another setback on Wednesday (AEST) when the defending champion was unceremoniously bundled out the Internazionali BNL d'Italia in his first match of the tournament against Fabio Fognini, just a day after his 30th birthday.

Fognini was a cut above the top seed, winning a one-sided second-round encounter 6-2 6-4 in Rome after Borna Coric consigned the three-time grand slam champion to an early exit in the Madrid Open last week.

Murray enjoyed a stellar 2016 – reaching three grand slam finals and winning Olympic gold in Rio – and said he is at a loss to explain his poor form, but Nadal thinks the Wimbledon champion can come good for the second major of the year at Roland Garros.

"He's a great player, unbelievable player," Nadal said. "He will be ready for Roland Garros. He has one week and a half to practice, and he will be one of the candidates [to win the French Open}."

"I don't think being number one has an affect on play ... I don't see a reason why. But at the same time, it's so difficult to be on the top of your game and mentality for every week.

"He had an unbelievable year last year, winning almost all the matches and that's tough. Physically, mentally, it's tough to do what he did at the end of his season."