Leimbach, 15, defeated the 16-year-old Collins, 4-6, 6-1, 6-2.
The fourth-seeded Collins combined heavy topspin with a wicked slice, both of which worked well in a first set that she took when Leimbach double-faulted on set point. It was her fourth of the set and the second time a service error cost her a game, but Leimbach wasn't discouraged.
"I stuck in there even though I lost the set because it wasn't over," she said.
Leimbach dominated the next two sets, playing the part of the patient counter-puncher. Collins would hit apparent winners, but Leimbach would run them down, smacking them back.
"I wanted to move her around," Collins said. "Instead I ended up hitting right back to her."