Alec Mills is a strike-thrower, and he saw no reason to back down from his aggressive approach on Tuesday night.

"When your team scores that many runs early, the best thing you can do is have quick innings and get them back up at the plate to keep the momentum going, so I was just trying to fill up the zone," the Royals' No. 14 prospect said.

Mills had a six-run lead after the second inning and capitalized on it. He gave up two hits and struck out nine over six scoreless frames as Double-A Northwest Arkansas completed a three-game sweep of visiting Midland with a 9-2 win.

The 24-year-old right-hander turned in his stellar start without ever locking in a great feel for his slider, curveball or changeup.

"The off-speed stuff was kind of fringy," he said. "It was kind of something I was trying to find all night. It just wasn't very consistent, so my fastball command was what I could rely on."

That fastball command is a large part of what's helped the 2012 22nd-round pick climb the prospect ranks. He took the loss in his Double-A debut last week, despite holding Frisco to one run on four hits over five innings. Mills has piled up 16 strikeouts against one walk in his first two Texas League outings.

"Pitch-wise, everything was very similar [about the two starts]," he said. "I got more into a rhythm tonight than I did on Opening Night. I pride myself on not giving out many free passes, but at the end of the day with a start like this, I can't be too unhappy about the one walk."

After allowing a ground single through the right side to A's No. 21 prospect Jaycob Brugman in the second and the lone free pass to Beau Taylor in the third, Mills fanned five in a row, striking out the side in the fourth.

"I didn't know it was five in a row," he said. "I knew I struck out the side, but I didn't realize it was five in a row."

Viosergy Rosa lined a one-out single up the middle in the fifth, but Mills immediately got back on track. When he finished the sixth by striking out Josh Rodriguez for the third time, the University of Tennessee-Martin product knew his evening was probably over. He'd thrown 81 pitches, 60 for strikes.

"I felt good and I could have gone back out there, but it's the beginning of the season," Mills said. "There's no need to push anything when we don't need to."

Top Royals prospect Raul Mondesi tripled, stole two bases and scored a run for the Naturals, while No. 7 prospect Bubba Starling also had a pair of thefts. He walked twice and scored three runs, despite going 0-for-1.

A's No. 6 prospect Matt Chapman walked and scored a run for the RockHounds.