Davis became the first player to hit grand slams for different teams in the same April, and Neil Walker had a run-scoring single with two outs in the ninth inning as the Pirates twice overcame deficits to beat the Cincinnati Reds 6-5 Monday night.
Pittsburgh trailed 2-0 before Davis' fourth-inning homer off Mike Leake. Davis hit a game-winning, ninth-inning slam off the Reds' J.J. Hoover on April 5 for the New York Mets, who traded him to the Pirates on Friday.
According to STATS, no player previously hit slams for different teams in the same April. Davis became just the third to hit slams for different teams against the same opponent in the same year, following Ray Boone in 1953 and Mike Piazza in 1998.
''We knew that's what he was capable of doing when we traded for him,'' Walker said. ''As a left-handed hitter in this ballpark, I can envision him hitting more balls over that Clemente Wall in right field. This is just the start.''
Leake said he made a good pitch and gave credit to Davis, his former college teammate at Arizona State.
''I broke his bat and he still hit it out,'' Leake said. ''That's how strong he is.''
With nobody out, Davis was just trying not to kill a rally rather than trying to hit a grand slam.
''I wanted something up because I know Mike has a good changeup and sinker so I didn't want to ground into a double play,'' Davis said. ''I was just looking for something I could hit into the outfield, either get a sac fly, base hit or, hopefully if I hit it good enough, a homer. I was just trying to get the ball in the air.''
The Pirates were behind 5-4 before Andrew McCutchen's leadoff homer in the eighth against Manny Parra.
Walker got the winning hit off Hoover (1-2) after consecutive one-out walks to Russell Martin and Andrew McCutchen followed by Pedro Alvarez's popout.
''It's totally on me,'' Hoover said of the loss. ''I hung a curveball on 1-2 count to Walker when I was trying to bury it in the dirt and I left it up a little bit where he could handle it. And you can't walk guys and expect to have good results.''
Jared Hughes (1-0) pitched a scoreless, two-hit ninth for the win after being recalled from Triple-A Indianapolis earlier in the day.
''I'm in Indianapolis yesterday and in Pittsburgh today getting a win,'' Hughes said. ''That's pretty cool.''
Cincinnati had taken a 5-4 lead in the eighth when Brandon Phillips and Todd Frazier singled off Francisco Liriano, Jay Bruce hit an RBI double against Justin Wilson and, after an intentional walk to Ryan Ludwick, Devin Mesoraco hit a run-scoring single.
McCutchen and Walker each had three hits for the Pirates, while Mesoraco, Phillps and Frazier had three hits each for Cincinnati. Mesoraco has a hit in all 10 games he's played this season and is batting 541.
Liriano gave up five runs - four earned - and seven hits in seven-plus innings. Leake allowed eight hits in seven innings.
NOTES: Boone, playing for Cleveland and Detroit, hit slams against the St. Louis Browns in 1953. Piazza, playing for the Los Angeles Dodgers and New York Mets, hit slams against Arizona. ... Pirates LHP Wandy Rodriguez was placed on the 15-day DL with right knee tendinitis, opening a roster spot for Hughes. ... Reds LHP Aroldis Chapman is tentatively scheduled to throw batting practice Wednesday, the first time he will face hitters since sustaining facial fractures March 19 when he was hit by a line drive during a spring training game. The closer had a 45-pitch bullpen session Sunday. . Cincinnati RHP Johnny Cueto (1-2, 1.50) faces Pittsburgh RHP Edinson Volquez (1-0, 1.71) on Tuesday night.