The Pirates, winners in 10 of their last 12, have a five-game lead over Milwaukee with six games remaining for both teams. Any combination of two Pittsburgh wins or Brewers losses will guarantee the Pirates at least a spot in the NL wild-card game for the second straight year.
The Pirates still have hopes of winning the NL Central, but they began the night 2½ games behind first-place St. Louis.
McCutchen, the 2013 NL MVP, hit his 24th homer in the sixth off starter Aaron Harang.
Liriano (7-10) allowed three hits, walked four and struck out seven in six innings. He extended his consecutive scoreless streak to 28 innings.