Former first overall pick Mark Appel is stepping away from the diamond.

Appel, who was selected with the top pick by the Houston Astros in 2013 but has yet to reach the majors, announced Thursday he's taking an "indefinite break" from baseball, in an interview with Joon Lee of Bleacher Report.

His decision comes after a tough 2017 season in the Philadelphia Phillies' system that saw him post a 5.14 ERA with 54 walks in only 84 innings across two levels.

"I'm 26, I have a Stanford degree, I have many interests beyond baseball, which I still love, but I have a lot of things I care about," Appel told Bleacher Report. "I enjoy challenging my mind. My last four years in baseball have challenged my mind.

"I don't know what the future holds. I'm pursuing other things, but also trying to become a healthy human."

Appel was drafted first overall by the Astros in 2013 out of Stanford - one spot ahead of Kris Bryant - and received a $6.35-million signing bonus. But after a decent introduction to the professional ranks, the decorated NCAA pitcher's struggles quickly mounted in 2014, when he posted a 6.91 ERA and allowed over 11 hits per nine innings in 83 1/3 innings split between High-A and Double-A in the Astros' system.

Though he reached Triple-A in 2015, Appel's numbers continued to disappoint across the board, and the Astros dealt him to Philadelphia that winter as part of a package for closer Ken Giles.

Injuries limited Appel to just 28 appearances over the past two years, the majority of which came at Triple-A level.

With this decision to take this "indefinite break" from baseball, Appel joins Steve Chilcott (Mets, 1966) and Brien Taylor (Yankees, 1991) as the only first-overall draft picks to have never played a game in the majors during their professional careers.