Kathy Hartwiger, the 2002 champion, fired a 2-under 70, the best score of the two stroke play rounds to move into the second seed at 3-over-par 147.
Kerry Postillion and Shannon Ogg, the first-round leader, shared third place at 5-over-par 149. Susan Roh, Susan Marchese and Claudia Pilot were one stroke further back at plus-6.
Probst recovered from a double bogey at the first with a birdie at the second to get her confidence back. She credited her caddy with help reading the tough greens at Holston Hills Golf Club.
'He plays golf too,' said Probst of her caddy, who is also her boyfriend. 'He knows what to say and when to say it. He leaves the decisions up to me.'
After dropping a wedge within one foot of the cup at the last, Probst then realized she finish as the medalist.
'That's really exciting,' said Probst.
Also among those advancing to the match play portion of this event are defending champion Amber Marsh Elliot and seven-time USGA champion Carol Semple Thompson.
Semple Thompson, a two-time winner of this event, moved on to the match play round for the 18th consecutive year, extending her own record.
Other former champions that advanced to the match play portion of the event include 1998 winner Virginia Grimes, 1988 champion Martha Lang and three-time winner Ellen Port.
The lone former champion in the field who failed to advance to match play was Robin Weiss Donnelley, the 1989 champion.
The first round of match play will take place on Sunday. The second and third rounds will be played on Monday, with the quarterfinal and semifinal rounds on Tuesday. The 18-hole final will be contested on Wednesday.
The Women's Mid-Amateur is one of 13 national championships conducted annually by the United States Golf Association, 10 of which are strictly for amateurs.