Twelve-times French Open champion Nadal was surprisingly beaten 6-2 7-5 by the Argentine in the Rome quarters -- Schwartzman's first win over the Spaniard in 10 meetings.
That was Nadal's first tournament for seven months, however, and he has progressed to the semi-finals at Roland Garros without dropping a set, although he was pushed hard in the early stages of his match against Jannick Sinner on Tuesday.
Schwartzman is through to his first Grand Slam semi-final after a five-hour victory over U.S. Open winner Dominic Thiem.
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"The match in Rome was very strange because Rafa won two very good matches against powerful opponents," Moya told Eurosport.
"Against Schwartzman it was the first time that he had problems in the score and he did not manage it totally well on a physical or mental level.
"We learned from that moment and now he is at a great level with many more games played. It will be a tough mental battle and I think we have some weapons that will allow Rafa to compete with guarantees."
Nadal will be the overwhelming favourite to avenge that Rome loss, although Moya says Schwartzman will provide a tough challenge and will not be affected by his attritional battle with Thiem.
"It will be a very difficult duel as we saw in Rome. Diego's match against Thiem yesterday was incredible and I think even Diego could have won more easily," Moya said.
"Diego is an incredible player and we don't think that the physical factor is going to influence too much."
Nadal has only lost two matches at Roland Garros out of the 100 he has played. (Reporting by Martyn Herman, editing by Ed Osmond)
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