Mitchell pitcher Patrick Schuster said he could tell during pregame warm-ups he didn't have the stuff that had led the Mustangs to a 22-3 record and four consecutive no-hitters.
But in the past, he has been able to overcome poor warm-ups.
The senior left-hander gave up three hits, walked five and hit three batters in five innings as Gaither took advantage of Schuster's control issues for a 9-4 victory in the Class 6A-District 7 semifinals at Countryside.
The Cowboys (21-6) will face top-seeded Alonso (24-2) for the fourth time this season in Thursday's district final.
Schuster's no-hitter streak came to an end in the third inning in front of nearly 1,000 spectators. After Schuster hit Mike Danner to lead off the inning, Drew Doty hit an 0-1 pitch over right fielder Steve Brown's head for a double.
"I knew when it left my hand, he was going to hit it," said Schuster, who has signed with the University of Florida. "I was getting my pitches up all day."
Schuster's line: 5 innings, 3 hits, 5 runs, 3 earned, 5 walks and 3 hit batters. He threw 100 pitches, 57 for strikes.
Alonso's Thomas Dorminy scattered two singles and struck out eight as the Ravens beat Countryside 2-0 in the other semifinal. Alonso got a run in the first on an RBI single by Matt Brandy and an unearned run in the fifth.
Countryside (13-14) got its first two batters on in the top of the seventh but Dorminy got a strikeout and the Ravens turned a double play to end it.