MILWAUKEE -- Mitch Keller showcased his big league potential on Wednesday in a 4-2 loss to the Brewers at American Family Field.

In his best outing of the season, the 26-year-old went 5 1/3 innings and allowed only one run on four hits, with seven strikeouts

“That feels really good to finally get results,” said Keller. “It’s been since Spring Training since I’ve gotten results, and it’s very relieving.”

Wednesday’s outing marked the third time in Keller’s career that he did not walk a batter and struck out at least seven.

“He was outstanding,” said manager Derek Shelton. “That was what we were expecting to see, him in attack mode. It was as good as I’ve seen him pitch.”

Keller was in attack mode all afternoon, throwing 18 first-pitch strikes to the 20 batters he faced.

“[It’s] just the mentality of getting ahead from the get-go,” said Keller. “Because when I get ahead, I know my stuff plays a lot better [than] from behind.”

Keller’s ability to command the zone early, especially with his fastball, proved to be the difference-maker. He leaned on the four-seamer on 68 percent of his pitches, the most this season. The four-year big leaguer went with it early to set the tone for the afternoon.

In the first inning, 10 of his first 13 pitches were fastballs as he worked a quick 1-2-3 frame.

His high fastball usage correlated to a 77 strike percentage, a career high. On 75 pitches, Keller threw 58 strikes. This was way up from his last two outings: 62 percent against the Cardinals and 60 percent against the Nationals. It was also up from his career average (62.7).

“I leaned on my fastball a lot today,” said Keller. “I thought it played very well. I think that’s the key for me going forward, just using my fastball, because everything works off of that.”

The fastball is Keller’s most dominant pitch, and it was working well for him. He drew seven whiffs, 11 called strikes, 16 fouls and reached 99 mph. But his lone earned run also came on that trustworthy pitch.

In the second inning, Rowdy Tellez sent Keller’s 96 mph fastball into the right-field seats. The ball was inside, but the Brewers’ first baseman turned on it after an eight pitch at-bat in which he saw six fastballs.

“Keller had a good pitch,” said Shelton. “It looked like Rowdy was looking for it and got it out front. It was just a pitch that a pretty good Major League hitter got to.”

Keller might have gone to the fastball one too many times, but that’s his pitch. Mixing in his offspeed stuff, however, will make the 26-year-old that much better for years to come.

“I think we lived in [that at-bat] a little bit too much,” he said.

The Pirates caught a glimpse of Keller’s potential to do just that earlier in the same plate appearance. Keller threw a fastball on the first two pitches, one of which Tellez fouled off. On the next pitch, he threw a low 79 mph curveball that Tellez also fouled off, but he was way out in front of it.

During his next at-bat against Tellez, Keller learned his lesson and threw three straight changeups in a four-pitch at-bat. Tellez grounded out.

Wednesday’s start was a vast improvement from Keller’s first two this season. He entered the game having given up 13 hits, eight runs and five walks with eight strikeouts over 7 2/3 innings.

“Nasty,” said Brewers catcher Omar Narváez of Keller. “Throwing 98 like nothing. Easy 98. Sometimes you kind of get a feel for somebody trying to throw hard and read body motion and how they load. This guy, it feels like he's going to throw 90, and then it jumps out the hand and gets on you quick."

The potential is there for the Iowa native, who was the Pirates’ second-round pick in the 2014 MLB Draft. If Keller can continue to thrive with his fastball and mix in other pitches more often, he can be the reliable pitcher that Pittsburgh always hoped he’d be.

“If he pitches like this the rest of his career, I’ll be one of the happiest human beings alive,” Shelton said.