Nadal is looking for a record-extending 13th title at Roland Garros and a 20th major overall to equal Roger Federer's men's record.
His 2020 debut at a place he's won 12 trophies did not change Nadal's view of things at this pandemic-postponed event: It's colder than usual, the balls are heavier than usual, he's had less preparation than usual.
"The conditions are completely different,'' he said, "than any other Roland Garros that we played.''
US Open champion Dominic Thiem, meanwhile, felt right at home in the cold of the French Open on Monday.
While most players have complained about the chilly weather conditions at Roland Garros, the third-seeded Thiem had no such concerns as he advanced to the second round by beating Marin Cilic 6-4, 6-3, 6-3.
The temperature on Court Philippe Chatrier dropped as low as 13 degrees Celsius (55 degrees Fahrenheit).
"I love the conditions because I'm from Austria, because I know how it is in these cold conditions," Thiem said on court. "Ten, 15 degrees, I love it.''
Daniil Medvedev is now 0-4 for his career at the French Open as runner-up at the U.S. Open last year and a semifinalist in New York last month failed yet again to win a match at Roland Garros. The fourth-seeded Russian lost to 63rd-ranked Marton Fucsovics 6-4, 7-6 (3), 2-6, 6-1.
Medvedev was in a bad mood throughout, getting warned for an expletive, then being docked a point for smashing his racket in the second-set tiebreaker. That point came at 6-3 in the tiebreaker, so he lost that set. Fucsovics had been 0-14 against Top 10 opponents.
Nadal did not face a break point in the first two sets but trailed 15-40 at the start of the third. He saved one but was broken when he hit wide down the line.
Gerasimov had a medical timeout for several minutes at 2-2. He received treatment on his right foot after an awkward fall. The foot was heavily strapped and his movement appeared limited afterward.
Nadal served for the match and won on his first match point when Gerasimov patted a shot into the net.
The second-seeded Nadal next faces American Mackenzie McDonald.
Later Monday, Tennys Sandgren won an 11-9 fifth set to become the seventh American man to reach the second round at this year's French Open -- the most in nearly a quarter-century. Sandgren had been 0-3 at Roland Garros for his career until saving two match points and coming back to edge No. 29 seed Hubert Hurkacz of Poland 7-5, 2-6, 4-6, 7-6 (1), 11-9 over 4 1/2 hours in a match that ended a little past 11 p.m.
Sandgren was a point from losing when he served at 15-40 while trailing 8-7 in the fifth. But he erased one match point with an ace, and the other disappeared when Hurkacz missed a running forehand. Sandgren then needed four match points of his own to close things.
Fourteenth-seeded Italian Fabio Fognini and 19th-seeded Canadian Felix Auger-Aliassime both lost in the first round of the French Open. Fognini lost 7-5, 3-6, 7-6 (1), 6-0 to Mikhail Kukushkin and Auger-Aliassime was beaten 7-5, 6-3, 6-3 by Yoshihito Nishioka.
Italian Lorenzo Giustino defeated Frenchman Corentin Moutet 0-6, 7-6 (7) 7-6 (3), 2-6, 18-16 in 6 hours and 5 minutes for the fourth-longest match in a major, in terms of time on court. It was also the second-longest in the history of the French Open.
The Associated Press contributed to this report.