Raducanu, 19, stormed into an early 6-3 lead in the first set, playing arguably her best tennis since winning the U.S. Open last August. But Sasnovich struck back to take the next set 6-1.
Raducanu went off court before the third, and had four break points in the third game to take an early lead. But Sasnovich held, and broke Raducanu twice to take the match 3-6, 6-1, 6-1.
"I thought that it was a pretty long match, pretty physical, as well," Raducanu said at a news conference on Wednesday. "I think my opponent, she played pretty well throughout, and didn't make any errors. But, yeah, I think that it was a long match for me, but, I mean, the whole clay season has been pretty positive overall, I would say."
This was Raducanu's first time playing in the main draw at Roland Garros, and she came through a tricky opening round tie against qualifier Linda Noskova on Monday, winning 6-7 (4), 7-5, 6-1. But Sasnovich -- who defeated Raducanu at Indian Wells last October -- proved to be too tricky a task for the British No. 1. Raducanu finished with 33 unforced errors compared to Sasnovich's 24, while Sasnovich hit 45 winners, compared to Raducanu's 17.
But it proved to be a valuable learning experience for Raducanu. She came into the clay season with low expectations, saying she expected to lose in the first round each time in the four tournaments she played pre-Roland Garros. But she rallied to win five of the nine matches from her first season on clay.
"I think I definitely got stronger as the clay season went on," Raducanu said. "It just takes a lot more to win the point on this surface, and, you know, you hit a ball flat and it doesn't really do that much.
"I definitely learnt like when to use the shape and stuff. Yeah, like I still got quite a long way to go on this surface, but overall, I would say I definitely had a good first experience on the clay. I think that I can definitely improve a lot more than what I am at right now."
The defeat on Wednesday on Court Suzanne-Lenglen squares off her first season on tour, given she started at Wimbledon last year. It was a remarkable year as she reached the fourth round of Wimbledon, and won the U.S. Open. She fell in the second round of the Australian Open after suffering from blisters, and exited at the same stage here in France.
"I think I do really welcome going around the second time," Raducanu said. "I think this year was always going to be challenging for me to adjust, find my feet. There's always something new. Like I'm always asking where everything is. I have no idea where everything is.
"It's going to be a lot more familiar this time around. I feel like in the last 12 months I have definitely grown a lot. On and off the court I feel like I have probably improved like how much I fight. I think that's one of my biggest strengths and even more so on the tour this year, and it's definitely opened my eyes to just how good everyone is and how much depth there is in the game.
"I think that it has been a pretty positive year just because I have learnt so much, and I think that the amount of learning that I have kind of done outweighs any sort of result, to be honest."
Raducanu will now turn attention to the grass court season, and to Wimbledon. She is due to play in the Rothesay Classic in Birmingham which starts on June 13, and will likely fit in another tournament or two before the championship starts on June 27.
"It's going to be really nice to go home and play on home turf, on the home grass," she added. "It is going to be a little bit strange in the beginning, because I have played on clay court for so long now, I feel like it's been weeks.
"I'm really looking forward to playing in front of the fans at home, and just all the support. Last year I experienced a little taste of it, but I feel like this year might be a bit more. I'm just really looking forward to the atmosphere that's going to be around."