The Reds, who had won just eight of their first 35 games in a start that cost manager Bryan Price his job, have been energized by the acquisition of Matt Harvey from the New York Mets, winning their last six games.
The most recent four wins came in a shocking road sweep of the defending National League champion Los Angeles Dodgers, with a suddenly rejuvenated Reds pitching staff allowing a total of nine runs in the four games.
Harvey pitched the second game of the series against Los Angeles, shutting out the Dodgers on one hit in four innings of a 6-2 win. He did not get a decision.
The former All-Star and Cy Young Award candidate is scheduled to pitch the finale of the series against San Francisco on Wednesday.
"It's a good first start," Harvey said after his Reds debut. "It's only four innings. There's a lot of work and a lot of season left."
The Reds had everything going in their 5-3, sweep-completing win over the Dodgers on Sunday.
Starting pitcher Luis Castillo pitched effectively into the seventh inning and reliever Amir Garrett struck out the side in the eighth to set up closer Raisel Iglesias' 1-2-3 ninth.
Right-hander Sal Romano will get the ball in Monday's series opener. He has never faced the Giants in his two-year career.
The 24-year-old has pitched well in his last two starts, allowing a total of two runs and seven hits in 11 1/3 innings in wins over Miami and the Mets.
The Giants got a season-best pitching performance from veteran left-hander Derek Holland in a much-needed 5-0 win at Pittsburgh on Sunday that capped a 4-6 trip.
Most important, it snapped San Francisco's six-game losing streak, making for a happier flight home, where the club will play its next seven games.
The Giants have won their last three home series.
"We needed that in the worst way," Giants manager Bruce Bochy said of the trip-ending win. "I said it would take a really good pitching effort to get us out of this. Give (Holland) credit. He took over and did a great job."
The Giants will call upon right-hander Chris Stratton to build upon any momentum gained against the Reds.
Like counterpart Romano, Stratton has never faced the Reds in his career.
Unlike Romano, the 27-year-old has struggled big time in his last three starts, getting roughed up for 14 runs and 17 hits in just 12 innings. The Giants lost two of those games by 15-6 and 11-3 counts.
Former Pirates star Andrew McCutchen had a successful individual homecoming effort in his first return to Pittsburgh since being traded during the winter, stroking four hits, including three doubles, in the three games.