ORONO – Laura Stein may have been undersized, unseeded and an underclassman, but she certainly wasn’t overmatched in Monday’s rounds of the girls singles tennis championships at the University of Maine courts here.

Stein, a Camden Hills freshman, knocked off top-seeded and 2000 finalist Chelsea Gray of Madison for the day’s biggest upset. Stein will face Waynflete of Portland’s Elizabeth Currie, the No. 4 seed, in the state semifinals Thursday at the Waynflete School.

“Going into it I was a little nervous. [Gray] was the top player and I wasn’t even seeded, “Stein said. “I knew she was a real powerful hitter and I knew I had to go in there and play defense and take advantage of the opportunities I had.”

No. 2 Erin Moakler, a sophomore at Greater Houlton Christian Academy, also advanced and will meet No. 3 Kelly Graham of Kennebunk in the other semifinal.

Moakler is the only returning semifinalist from last year’s group. She lost to Gray in the semis last year.

But that won’t happen this year, thanks to Stein.

The 5-foot-2 Stein, Camden’s No. 1 singles player, opened with a 6-2, 6-1 win over Lisa Shaw of Portland’s Catherine McAuley. In the next round she defeated eighth-seeded Molly Gilbride of Mount Ararat in Topsham.

That set up her meeting with Gray. Visually, it seemed Gray would have a clear advantage. The Bulldog senior stands about 5-9 and is a muscular heavy hitter. Stein admitted to being a little intimidated at first.

But Stein broke Gray on her first service and took a 4-3 lead in the first set. Gray got back into it and went ahead 5-4, but Stein held on and beat Gray 7-5.

Gray opened the second set with a 1-0 lead, but Stein tied it up and was ahead for the rest of the match.

“I was just trying to get the ball back and let her make mistakes,” she said. “It was hard off of her pace to take control of the point right away. That worked to my advantage.”

Stein was hoping to get in a lot of first serves so Gray wouldn’t be able to overpower her second serve.

John Lippman, the tennis pro at the Megunticook Golf Club in Camden who works with Stein, said the freshman has enough experience from New England tournaments to know when to pounce.

“It was a high percentage of first serves and few unforced errors, and I think that’s what got her through,” he said. “Unfortunately Chelsea made some errors and lost her focus and composure. Laura knew the girl was in trouble, and she kept her focus.”

Stein wasn’t lacking in on-court presence. She grunted during her returns like a young Monica Seles and punctuated her matches with slaps on her leg and shouts of “Come on!” and “Let’s go!”

With Gray knocked out of it, Moakler, one of the top-ranked youth players in the state, has to be a favorite for Thursday’s semifinals and finals. She rolled through her matches Monday.

Moakler opened the day with a 6-1, 6-0 victory over Emily Boyle of Waterville and moved on after a 6-2, 6-1 defeat of Lewiston’s Amy Sarrazin.

Moakler lost the opening game of her match against Boyle, but rebounded on the strength of some well-placed cross-court strokes.

“I was nervous at first, but after that I was fine,” she said. “It just takes me a couple of games to get used to it, and then after that I was fine.”

Moakler beat No. 7 Robin Sewell of Orono in their quarterfinal meeting 6-0, 6-0. Sewell had just come off a three-set victory over Lissa von Brecht of North Yarmouth Academy.