Sharapova, a five-time Grand Slam winner and former world number one, lost 3-6, 7-5, 6-4 to Kristina Mladenovic in a gruelling two-hours, 38-minute showdown.
"I'm not angry, I'd have loved to have used the opportunity when I was ahead in the second set, so I had a bit of a let down which allowed her to get back in the match, gain confidence and play well," said Sharapova, who was playing her first tournament after testing positive for meldonium at the 2016 Australian Open.
Sharapova also has wild cards for the Madrid and Rome tournaments and leaves Stuttgart with a world ranking of 260 having arrived here on zero following her ban.
She will learn on May 16 whether she will be given a wild card for the main draw at the French Open where she has twice been champion.
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Sharapova jets to Madrid to prepare for the WTA tournament in the Spanish capital and will play in an exhibition match on Thursday.
"I'll be in Europe for the next couple of months, but this is what I want to go through.
"I want the matches and to see how my body responds. I want to feel the tiredness of match play, it's different and you can't train that.
"I love that feeling. This is what I asked for, this is what I am doing."
"I am treating Madrid and Rome just as importantly as the Grand Slams at this point, because I have been out of the game for so long. They are match play and that is what I need."
"If at the start of the week I'd said I'd be in this position, I'd be pretty happy with that," she said.
"The way I played, I was really happy with that. You are never sure what level you are going to come onto the court with, but I feel this is a great base with which I started here.
"I actually felt physically really well, even when I was 5-2 down in the third set.
"I still hung in there, but she came up with a string of good points which cost me the match."