Sharapova, who last played a singles match in July after being sidelined with a torn rotator cuff in her right shoulder, wore a bandage on the arm but said the shoulder didn't bother her.
She cruised through the first set and grabbed a quick 4-0 lead in the second before her serve started to falter. Serving at 5-3, she wasted four match points - double faulting on two of them - and then failed to convert two more in the tie-break before netting a forehand to give the set to Garbin.
In the third, the Russian dropped an early break before rallying with her trademark groundstrokes to overpower the Italian. Sharapova held serve to go up 5-3, then converted her third match point when Garbin knocked a backhand long.
Once ranked No. 1 in the world, Sharapova's ranking has slid to No. 126 since her injury forced her to miss the last two Grand Slams.
Sharapova refused to speculate about next week's French Open, saying she was only thinking about Warsaw this week. But she stressed that playing matches was the only way to return to her championship form.
"I've been absent for so long, and I've said it many times, you can do so many things, you can practice and you can play practice matches, but it's never the same as going out and playing in a tournament, and I think that's what I'll need," she said.
"I've played millions of matches in my career, and I'll play millions more, and I think right now it's just going to be getting that experience back and the thought process on the court and doing the right things to finish the match."
Thoughts: How similar are Sharapova's comments about playing her best tennis to doing doing well in an examination?
