SPRINGFIELD, Ill. — Michelle Wie finished the third round of the State Farm Classic alone in second today — then was disqualified for failing to immediately sign her scorecard a day earlier.

Wie, the 18-year-old from Honolulu, was playing her best golf of the year, finishing off a 5-under 67 to get to 17 under for the tournament, one stroke back of Yani Tseng.

That's when Wie was disqualified by LPGA officials, who said they'd learned during play today from tournament volunteers about the 18-year-old's mistake.

"I don't know why or how it happened," said Wie, who had been crying moments earlier. She took no questions before leaving the clubhouse at Panther Creek Country Club.

Sue Witters, the LPGA's director of tournament competitions, disqualified Wie in a small office in an LPGA trailer at the golf course after asking her what had happened.

"I felt like I was telling somebody that there was no Santa Claus," Witters said.

Wie said that after she finished her round on Friday, she left the tent where players sign their scorecards and was chased down by some of the tournament volunteers working in the tent who pointed out she hadn't signed.

Wie returned to the tent and signed the card.

"I thought it would be OK," she said.

But Wie, according to Witters, had already walked outside the roped-off area around the tent. At that point, the mistake was final.

Witters said she and other tour officials didn't learn about the error from volunteers until well after Wie teed off today. They let her finish the round, then took her to the office where she and her caddy, Tim Vickers, were informed of the ruling.

Wie, who is playing a part-time schedule while attending Stanford, opened with a 5-under 67 and followed with rounds of 65 and 67 — though the last two won't count.

It's not the first time a mistake has cost Wie, either. She was in fourth place during the third round of the 2005 Samsung World Championship — her professional debut — when she was disqualified for taking an improper drop.

Tseng shot a 6-under-par 66 today to snatch the lead from Christina Kim, who entered the day atop the leader board. Tseng is 18 under for the tournament, and didn't bogey a hole while sinking four birdies and an eagle.

Kim shot a disappointing 73, finishing the day in a tie for eighth place, six behind Tseng.

With Wie's disqualification, Katie Futcher finished the day second place at 16 under. She shot 66 today.