MILWAUKEE — Garrett Mitchell hit his first career grand slam as the Milwaukee Brewers overwhelmed the Cincinnati Reds 20-0 Friday night, securing a postseason berth with one of their most dominant performances of the season. The Brewers scored in bunches and finished with a season-high 23 hits. Mitchell went 3 for 5, while Jackson Chourio collected four hits and two RBIs. William Contreras, Bryce Turang and Jake Bauers also homered for Milwaukee. Milwaukee improved to 92-56 and guaranteed at least a wild-card spot. The Brewers hold a 10-game lead over the Chicago Cubs in the National League Central, with four more wins needed to clinch the division. Mitchell’s slam capped an eight-run fifth inning against Cincinnati reliever Luis Mey. Turang went 3 for 3 with a walk and hit his 18th home run, a three-run shot during Milwaukee’s six-run second inning. Bauers followed with a two-run homer that traveled an estimated 412 feet to center field, his 25th of the season. Contreras added a three-run homer in the eighth inning after Reds catcher Jose Trevino took the mound. Trevino retired Milwaukee in order in the seventh before giving up the long ball in the following inning. Milwaukee’s pitching staff also delivered a shutout. Dustin May allowed only two hits over six innings, both singles by Cincinnati rookie Sal Stewart. Garrett Stallings recorded the final nine outs for his first save. May, who was acquired from St. Louis at the trade deadline, earned the win to move to 7-9. Cincinnati starter Andrew Abbott fell to 6-11 after surrendering eight runs on nine hits in two innings, his shortest start of the season. Reds shortstop Elly De La Cruz walked in the sixth inning to extend his streak of reaching base safely to 18 games. Cincinnati will start right-hander Brady Singer, who is 6-13 with a 5.03 ERA, in Saturday night’s game. Milwaukee had not announced its starter.