Nadal, 32, lost his opening service game and trailed 2-0 in the first set before recovering well in Paris.
Nadal responded to reel off five straight games to assert his dominance.
Having gone two sets up on the back of big hitting from the baseline, sixth seed Anderson threw Schwartzman a lifeline in the contest while serving for the match at 5-4.
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Nadal quickly rallied before securing the tiebreaker on his second match point when Marterer fired long, celebrating with a trademark fist pump.
But Marterer, who was playing in the main draw here for the first time, moved into a 3-1 lead early in the third as he threatened to become the first man to take a set off Nadal at Roland Garros since his 2015 quarterfinal defeat by Djokovic.
But he kept fighting, forcing a tie-break only for Nadal to up the intensity once again and open up a 5-2 lead.
Pakistan beaten by an innings as England level series
One eagle-eyed viewer spotted a motivational message written on the handle of Buttler's bat today, that appears to say "fuck it". Anderson used favourable bowling conditions effectively with his twin strikes before lunch as Pakistan wobbled at 48-3.
Next up for the Spaniard is a last-eight meeting with Diego Schwartzman, who came from two sets down to see off Kevin Anderson, runner-up to Nadal in last year's US Open final.
The first went begging with a forehand long of his own, but Marterer's fight was finally over when he could not land a backhand, prompting Nadal to race to the net and congratulate the German on a fine effort.
Marterer was only the 11th man in 50 years of professional tennis to make the last 16 on debut at Roland Garros, and first since Nadal won the first of his record 10 titles in Paris in 2005.
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