The Russian, a three-time Grand Slam champion, struggled at times in her first tournament since October's China Open and finally met a player in Arn, 31 years old and unseeded, who could make her pay for her mistakes.
"I'm like a little girl whose dream has come true," Arn said.
Sharapova's serve was shaky in three matches this week -- the former world No. 1 particularly struggled with her ball toss in windy conditions -- and showed a lack of patience and accuracy in rallies.
Her coach counseled her during her first- and second-round matches to play through the middle of the court and take her time to work her way into points but Sharapova usually looked for the quick winner.
She tried to hit the lines with her powerful forehand but was missing her usual accuracy. She finished with 30 unforced errors.
"I would have loved to play a few more here and be the winner," Sharapova said. "But that's the way it goes. You look forward to the next one. That's the good thing about tennis."
The contrast could not have been greater between Sharapova and No. 2 seed and defending champion Yanina Wickmayer, who breezed through her quarterfinal against Simona Halep of Romania 6-0, 6-2 in 63 minutes.
Wickmayer won the first nine games and Halep allowed herself a fist pump when she held serve for the first time in the fourth game of the second set.
Wickmayer's semifinal opponent will be Peng Shuai of China, who advanced with a 6-4, 7-5 victory over British qualifier Heather Watson.