Sisler began the 1958 spring training in spectacular form, pitching a total of nine hitless innings and allowed just one baserunner in 28 batters faced; the last 27 in order. However, he finished the spring with two consecutive losses, including one to the Pittsburgh Pirates, with whom his father was a special assistant to their manager. He opened the regular season with a complete game victory against the Yankees on April 16. On May 2, he pitched his only career complete game shutout, a 6–0 victory against the Detroit Tigers and future Hall of Famer Jim Bunning, improving his record to 3–1 for the season. Over the next two months, his average innings pitched declined during his starts, and on several occasions he entered the game in relief. He went winless from June 12 to August 9, when he defeated the Yankees. He recorded just one more victory in 1958, and completed the season with an 8–9 record, a 4.94 ERA, and 71 strikeouts in a career-high 149⅓ innings pitched.