Castillo struck out nine and walked one hitter across 7 2/3 innings in the Reds' 1-0 victory over the Brewers at Great American Ball Park. The 26-year-old ace's only blemish was Keston Hiura's one-out single in the top of the seventh inning.
"I think it's the best outing in all my career," Castillo said. "I felt really good out there."
Castillo is 8-3 with a 2.29 ERA in 18 starts this season. He was 0-2 with a 6.08 ERA in three starts against the Brewers this season before taking the mound Thursday.
"With Castillo, you have to pick a pitch, the fastball or the changeup, and he did a good job of keeping our hitters guessing," Brewers manager Craig Counsell said.
Castillo had two outs in the eighth when play was halted because of heavy showers. Reliever David Hernandez took over and walked pinch-hitter Jesus Aguilar, who had taken two balls from Castillo before the delay.
Castillo said he was disappointed he did not return to finish the game.
"No runs, one hit, I was throwing the best game in my career," he said. "But those things I cannot control."
The Reds scored the game's only run on Yasiel Puig's RBI single in the bottom of the first inning.
Only 3 1/2 games separate the first-place Brewers (46-42) and last-place Reds (41-44) in the NL Central Division.