Arizona lost its third straight game on Saturday, as Orlando Hudson hit the go-ahead homer in the fourth inning, propelling San Diego to a 3-1 win.
"It's disappointing," said manager Kirk Gibson. "But we have nobody to blame but ourselves -- and we're the people who can fix it as well, and we will."
Jeremy Hermida added a solo shot and Andy Parrino hit an RBI single for the Padres, who have taken the first two games in the series after suffering a three-game sweep in San Francisco.
Wade LeBlanc (4-5) scattered four hits and one run over six strong innings for the win.
Aaron Hill's first-inning homer was the lone offense for the D-Backs, whose lead in the division has been dwindled to five games thanks to the Giants beating Colorado on Saturday.
"It's still looking good," catcher Miguel Montero said. "We've just got to play a little better baseball. We've got to come back tomorrow and do a little better."
Josh Collmenter (9-10) took the loss, charged with eight hits and three runs over seven full frames.
Saunders gets the ball today trying for his third straight win. He beat the Los Angeles Dodgers on Monday, holding them to a pair of runs and four hits in seven innings, as he ran his record to 11-12 on the year to go along with a 3.83 ERA.
Saunders beat the Padres the last time he faced them and is 4-3 in seven starts against them with a 3.54 ERA.
San Diego, meanwhile, will rely on veteran right-hander Aaron Harang, who is 13-6 with a 3.85 ERA. Harang lost for the third time in four starts on Monday in San Francisco, as the Giants reached him for four runs and six hits in five innings.
Harang is 3-5 lifetime versus the D'Backs, despite a sparkling 1.91 ERA in 12 games (11 starts).
Arizona has won 10 of its 17 meetings with the Padres this season.