A four-time winner of this event, Semple Thompson finished her two rounds of stroke play at 7-over-par 151, but it was good enough for a one-shot victory over two players.
Her overnight lead withstood the completion of play Monday morning after the second round was suspended on Sunday because of darkness.
'What's important to me is to make it into the match-play round,' Semple Thompson said after her second round. 'I've achieved that objective.'
The 56-year-old Semple Thompson won the last of her four Senior titles in 2002 at Allegheny Country Club in Sewickly, Pennsylvania -- her home course. That was her fourth straight victory in this event.
But winning isn't as easy as she has made it seem.
'Match play can be fickle,' Semple Thompson said. 'You have to be a bit lucky to make it through six rounds of match play.'
Diane Lang and Therese Quinn finished the stroke play portion in a tie for second place at 8-over-par 152.
In addition to Semple Thompson, three other previous champions advanced to match play, including defending champion Carolyn Creekmore, who finished tied for 38th place at plus-19.
Jean Smith, the 1995 champion, and three-time winner Marlene Stewart Streit (1985, 1994, 2003) also advanced.