Nadal's last win after losing the opening set came against Novak Djokovic in the 2014 final.
The world No. 1 found himself a set and a break down to Diego Schwartzman, with his French Open hopes hanging in the balance, before rain initially stopped play. Schwartzman saw two set points come and go but at the third time of asking, after a delay while a spectator was helped out after feeling ill, the Argentine ripped a forehand victor.
The 25-year-old Schwartzman switched gears in the tie-break of a tight fourth set to level the contest at two sets apiece as a tiring Anderson failed to score a point.
Nadal made 14 unforced errors and struck just four winners in a poor first set, before having both of his wrists strapped by the trainer.
When they return to the stadium, Nadal will be serving at 5-3, 30-15 in the second set.
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The Spaniard, who won five straight games after a rain delay to beat Alexander Zverev in the Rome Masters final last month, managed three in a row this time before play was ended for the day.
But as in the first set, Nadal's riposte was swift, chasing down a drop-shot to set up a break-back opportunity, converted when Marterer thumped a forehand long.
The players were forced off the court for 40 minutes, but when they returned Nadal levelled for 3-3 and let out a trademark roar of vamos.
Eleventh seed Schwartzman snapped Nadal's run of winning 37 consecutive completed sets at Roland Garros when he took the opener 6-4 on Court Philippe Chatrier.
Halep won in 59 minutes with the second set taking just 22 minutes.
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