Three batters later, Jordy Mercer singled. All three of Pittsburgh's early hits came at the expense of Ross' slider.
Even without his best pitch, Ross figured it out.
"I just didn't have a good feel for my slider pretty much all day," Ross said. "I gave up three hits in the first inning, all on sliders that were up in the strike zone, and it's kind of out of the norm for me."
San Diego's win clinched a 14th-consecutive series victory at PNC Park.
Ross (11-10) allowed Gregory Polanco's two-run shot in the first then shut down the Pirates. He yielded six hits and walked two.
"I thought he battled without his best stuff and his best command," San Diego manager Bud Black said. "Hanging slider to Marte to lead off the game and then a hanging slider to Polanco, I think that kind of woke him up a bit."
"In the fourth inning, the sinker away to left-handers starting elevating," Pittsburgh manager Clint Hurdle said. "A couple left-handers were able to get balls out over the plate and hit them to left field."
That happened when Morton left a 2-0 fastball over the plate and Smith tripled to left for three RBIs and scored on a wild pitch for a 5-2 lead.
Morton (5-11) was lifted after the fifth, responsible for five runs, five hits, a walk and five strikeouts.
"It was almost like two outings for him," Hurdle said. "One through three and then four and five, it's just two different sides of it."
Pirates: Second baseman Neil Walker (back spasm) was scratched from the lineup. He pinch-hit Saturday night and was slated to start Sunday. ... Center fielder Andrew McCutchen (rib) was out of the lineup for the sixth straight game. ... RHP Gerrit Cole gave up one hit in six shutout innings Sunday as he made his third rehab start for Triple-A Indianapolis.
Pirates: Pittsburgh continues its eight-game homestand with a two-game interleague series against the Detroit Tigers. LHP Jeff Locke (3-3, 3.78 ERA) opposes Detroit RHP Justin Verlander (10-10, 4.57 ERA).
Since PNC Park opened in 2001, the San Diego Padres have never lost a series in Pittsburgh. Their 14-series winning streak at PNC Park is the longest active streak against a single opponent in a single location in the league.
Overall, the Padres are 33-12 in Pittsburgh since 2001.
"We're just playing good baseball," Black said. "Maybe the times of the year we come in here, we're playing well and we're pitching well."