Eric was selected by the Milwaukee Brewers in the 12th round with the 395th overall pick of the 2012 Major League Baseball Draft. In 2012 Semmelhack recorded five victories in 15 starts on the season, marking the second-highest start total in program history. He recorded 72 strikeouts (sixth all-time in a single season) in 88.1 innings, posting a 3.77 ERA and a .265 opponent batting average. Semmelhack finished first in the Horizon League in strikeouts and strikeouts looking, second in starts, fifth in opponent average, sixth in ERA, seventh in innings pitched and tied for eighth in wins. His career ERA of 3.94 ranks fifth all-time at UWM and also finished sixth with 153 career strikeouts and in the Top 10 of innings pitched and starts despite playing just three seasons.

In high school, Semmelhack went 10-4 on the mound as a senior, leading his team to a divisional title in the Greater Metro Conference and a spot in the state tournament. He set a school record by throwing 85.1 innings, while his 10 wins and 97 strikeouts were second all-time. Semmelhack had a 2.21 ERA as a junior, limiting opponents to just a .194 average. He earned a spot on the Milwaukee Journal Sentinel All-Area Honorable Mention Team as a utility player and was also named First Team All-Division Greater Metro Conference twice. Semmelhack also earned a 2008 CNI All-Suburban High Honorable Mention and selected to play in the WBCA Brewers Classic All-Star Game.