The 20-year-old, who won his first Masters title against Novak Djokovic last month, went down to the experienced Spaniard in four sets. Top seeds Andy Murray and Stan Wawrinka eased into the second round.
Germany's ninth-seeded starlet Alexander Zverev was stunned by Spanish veteran Fernando Verdasco, who secured a 6-4 3-6 6-4 6-2 win in the first round of the French Open.
After becoming just the fourth German to win a Masters crown, Zverev was highly fancied at Roland Garros after winning the Rome Masters and reaching the final of the Geneva Open last week.
A combination of rain and bad light put halt to the 20-year-old's comeback last night as he bounced back to take the second set 6-3 before he agreed to Verdasco's request to postpone play.
But Zverev struggled to find any rhythm after the restart of play and the more experienced Verdasco took full advantage, forcing 50 errors from the German.
"I played absolute crap. That's why I lost," the youngster said. "But life goes on, it's not a tragedy. In Rome I played fantastic, I won the tournament. Here I played bad, I lost first round. That's the way it goes."
Left-handed Verdasco, who has won six of his seven titles on clay, will meet France's Pierre-Hugues Herbert in the second round as he looks to make the last 16 at Roland Garros for a fifth time.
"I practiced really hard before this match against a great player like Alexander and I am super happy to be here on this court," said the 33-year-old, appearing at this 14th French Open.
"Maybe it was my experience. You have to be ready emotionally and physically because it can be a long match."
Stan Wawrinka began his quest for a fourth Grand Slam title with a 6-2, 7-6 (8-6), 6-3 win over Slovak number 152 Jozef Kovalik.
The 32-year-old committed a number of unforced errors as he made heavy weather of the second and third set. But he maintains his perfect record against players outside of the top 50 and will meet Alexandr Dolgopolov for a spot in the last 32.
"I enjoyed it very much," said Wawrinka, who won the French Open in 2015 and is current US Open champion. "It was not necessarily easy after I played in Geneva until Saturday to get into gear. I'm feeling good, I'm playing good tennis and I'm happy to be back in Paris."
Meanwhile, world number one Andy Murray cruised into the second round with a dominant 6-4 4-6 6-2 6-0 win over Russia's Andrey Kuznetsov.
Last year's beaten finalist, Murray's form took a worrying dip in the months before Roland Garros with the Scot admitting to a drop in motivation since he assumed top seed in the men's rankings. After losing in the fourth round of the Australian Open, Murray was knocked out of the Indian Wells Masters, the Monte Carlo Masters, as well as, the Madrid Open and Italian Open.
After a few nervy moments, Murray finished off the final set in just 27 minutes, winning eight successive games. "Last year was a great year for me, you know it was the best I ever played," Murray said. "Here at the beginning of my career I struggled. But each year I kept coming back and was trying a little bit better and last year was really good."
The three-time Grand Slam champion will take on Martin Klizan of Slovakia for a place in the last 32.
rd/ (RTRE, AFP, dpa)