Seven-times champion Nadal let his feelings be known after Parisian downpours and lop-sided scheduling left him with only one day's rest before facing the showman Italian, who has had an extra 24 hours recuperation.
After being taken to four sets by Daniel Brands in round one and Martin Klizan on Friday, Nadal expects Fognini, with his flashy winners, to be in better shape.
"Probably my opponent is better than me, talking about physical performance, but the only thing I can say is I'm going to work as much as I can to be 100 per cent ready," Nadal said.
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"It will be a really tough match, and I need to play better than today. I need to do it."
In contrast to Nadal, Novak Djokovic looked imperious in his second-round victory over Argentina's Guido Pella, but in Grigor Dimitrov he faces a player who beat him in the second round of the Madrid Masters this month.
The world number one is keen to see if the Bulgarian can back up his obvious potential.
"A few tournaments does not change much," Djokovic said. "Of course it can help him mentally, for his confidence and self-belief on the court, because he's got shots, he's got talent."
Women's champion Maria Sharapova faces China's Jheng Zie, and third seed Victoria Azarenka opens the action on Philippe Chatrier Court against France's Alize Cornet.
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