Barker (2-0) matched a season best with eight innings pitched. The sophomore right-hander allowed six hits and one run (earned), with one walk and one strikeout.
"He did a really good job of locating pitches and changing speeds," MSU sophomore shortstop Adam Frazier. "We couldn't get a timely hit. You have to tip your hat to him. We didn't come out and play like we should have.
"We found some barrels but couldn't get the big hit. Credit to their pitcher because he is trying to get double plays. He got his job done."
The Bulldogs hit into three double plays and managed seven total hits in the contest. MSU stranded six base runners and did not score until trailing 3-0 in the sixth inning.
The loss snapped a season-best seven-game win streak for the Bulldogs. Mercer improved to 12-4, while MSU fell to 13-3. The rubber game of the weekend three-game series is at 1 p.m. Sunday.
"Nick (Routt) and Evan (Mitchell) didn't have their best games," MSU head coach John Cohen said. "However, we have pitched it well in 15 of 16 games this year. Trevor (Fitts) and Chad (Girodo) pitched well in relief. If you look at the number of players who are freshmen and sophomores on this team, you realize we are on a learning curve.
"Every day, you are trying to get better. Chalk this up to a good learning experience."
For MSU, Routt (1-1) started and was lifted after a leadoff single in the fifth inning. The Bears reached their leadoff man six of nine at-bats Saturday. Routt struck out six, while allowing nine hits and three runs (two earned).
Mercer struck for single runs in the first, third and fourth innings.
MSU finally scored on a sacrifice-fly by Wes Rea in the home half of the sixth inning. The sixth inning threat was MSU's best since Frazier and Demarcus Henderson led off the first inning with back-to-back singles.
With those hits, both Frazier and Henderson extended hit streaks to seven straight games.
Mercer then touched Mitchell for three runs in the seventh inning, including solo home run by Logan Gaines and a two-run double from Evan Boyd.
"You can't anything away from Mercer," Cohen said. "They have a fine offensive ballclub. It seems like the ball sometimes goes further off their bats than others. They put you in a position to have to make good pitches because they take really good approaches."
Nick Backlund had four of Mercer's 14 hits. That hit total was the most allowed by MSU pitching this season. Boyd had three hits, while Gaines had two hits.
Frazier and Mitch Slauter each had multiple hits for MSU.