Spaniard Nadal, bidding for an 11th title on the Parisian clay, also trailed by a break in the second set as Schwartzman threatened a huge shock, but after a rain stoppage, Nadal returned to court to move 5-3 ahead in the second set.
Nadal had to receive treatment to his wrist at the end of the set with extra strapping applied.
The enforced interlude had clearly benefited Nadal, and assuming he gets the two points he needs to level the match he will start a strong favourite on Thursday.
The victor will face either Juan Martin del Potro or Marin Cilic, who were locked at 5-5 in a first-set tiebreak.
Schwartzman, who had never even reached a Grand Slam quarter-final until last year's US Open, broke first in the second set as Nadal uncharacteristically blew a 40-15 lead, capped by two wild forehands.
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Defending champion Rafa Nadal celebrated his 32nd birthday on Sunday making short work of Richard Gasquet ( 6-3, 6-2, 6-2 ) in his third-round match, and then made a ball boy's day at the Roland Garros.
Nadal's last win after losing the opening set came against Novak Djokovic in the 2014 final.
"I was up against a great player so I had to make sure I brought my best tennis".
Halep won in 59 minutes with the second set taking just 22 minutes.
Nadal, returning with his famed forehand finally firing on all cylinders, earned back-to-back breaks to move within a hold of the set.