Peavy (2-5, 4.30 ERA) is 0-4 with a 4.50 ERA in his last five starts since beating the New York Mets on April 16. The 2007 NL Cy Young Award winner went 0-5 with a 6.61 ERA over eight starts from June 6-July 21, 2006.
In his last appearance, Peavy pitched with the lead for the first time since his most recent win, but allowed three runs and six hits while striking out nine in six innings of a 6-2 loss to the Chicago Cubs on Tuesday.
"Just good enough to lose," he told the Padres' official Web site. "I believe in my ability and what I'm able to do. I'm going to stick with it."
While Cincinnati (20-16) is off to a strong start, the right-hander is 5-0 with a 2.31 ERA in nine starts against the Reds. The Padres are 8-1 in those games.
On Saturday, Nick Hundley homered with two outs in the bottom of the 16th inning to give San Diego (15-22) a 6-5 win in a game that lasted 5 hours, 14 minutes - the longest in the majors this season in terms of time and innings.
"I was just trying to enjoy the moment," Hundley said. "That was special."
Having already secured their first winning series since taking two of three at Philadelphia from April 17-19, the Padres are in position for their first sweep since winning all three versus San Francisco at home from April 10-12.
Scott Hairston had three hits to extend his hitting streak to nine games and raise his average to .340 for San Diego, which hasn't swept a home series from Cincinnati since June 16-18, 2000.
Adrian Gonzalez extended his hitting streak to seven games, but failed to homer for the first time in six contests. The Padres first baseman still leads the major leagues with 15.
"We're seeing the emergence of one of the best players in the National League," manager Bud Black said of Gonzalez, who is batting .321 (9 for 28) during his streak, but 2 for 15 against the Reds' Bronson Arroyo.
Arroyo (5-2, 7.02) gave up five runs and 10 hits - including three homers - in seven innings of a 13-5 win at Arizona on Monday. The right-hander has yielded 14 runs in eight innings spanning his last two starts.
"My ERA is up, and I've got more wins at this point than maybe any other year except 2006 (when he started 5-1 through seven starts)," Arroyo told the Reds' official Web site. "You take what they give you, and I'll keep taking wins."
Arroyo is 4-4 with a 3.97 ERA and has won two in a row against San Diego.
Jay Bruce hit a two-run homer for his 11th of the season Saturday for the Reds, who look to avoid a third consecutive loss after sweeping a three-game set at Arizona to open a six-game road trip.
Cincinnati will be without slugger Joey Votto after he left Saturday's game after because of some mild dizziness that forced him to miss Wednesday's 10-3 win over the Diamondbacks. Votto is among the NL leaders with a .366 batting average.