SAN DIEGO -- Eric Stults had nothing to show for his best start of the season, though it was just what his team needed.
Venable's third game-ending hit scored Chris Denorfia from third to complete an improbable comeback after Padres batters struggled mightily through the first eight innings.
It also allowed Stults to smile after a career-high, 12-strikeout performance where he allowed one run and three hits over eight dominant innings.
"The numbers were great, but the win was the important thing," Stults said. "The end result is all that matters and I'm proud of how we rallied late and were able to finish this game on top."
Kyle Blanks forced extra innings in the bottom of the ninth with a one-out sacrifice fly off Mariners closer Tom Wilhelmsen, who blew his second save in 13 tries by allowing just his second earned run in 23 innings.
Stults, by contrast, was on point.
"Eric pitched brilliantly," Padres manager Bud Black said. "Stults has pitched some very good games for us, but I don't know how you throw any better. All four pitches were effective and were thrown with excellent command. That was impressive."
The left-hander struck out hitters in bunches, including two stretches of four straight. Stults' previous strikeout best was nine, set on Aug. 17, 2007, while pitching for the Los Angeles Dodgers against the Colorado Rockies.
"I'm not really a strikeout guy," Stults said. "I normally pitch to contact, but I felt like I could attack their aggressiveness and use all my pitches to get them out. I was able to throw off-speed pitches low in the zone, and I believed I could make them swing and miss with that game plan."
The Mariners were frustrated all night by a pitcher who doesn't throw fastballs above 90 mph and has a curveball 20 mph slower. They've lost seven straight on the road, and have struggled to score runs during a brutal 2-10 stretch.
"We have got to get the offense going," Mariners manager Eric Wedge said. "That's what sticks with you during runs like this. We're not scoring enough runs and we're inconsistent. The guys have to step up."
Seattle wasted a solid effort by starter Joe Saunders.
Saunders continued his dominance over the Padres with one run and four hits allowed in seven strong innings, but his bullpen let him down. Saunders has allowed just one run in his last 29 2/3 innings against the Padres.
Chase Headley opened the scoring with a 403-foot solo home run to left field in the first inning.
Saunders kept the Padres off balance with a mix of pitches and a well-placed fastball, and pitched 6 1/3 shutout innings after Headley's blast.
"When you see a guy like Stults putting up zeros, you tell yourself you have to do the same," Saunders said. "Whatever it takes, you have to keep your team in the game."
Saunders did so, but the Padres wore on the Mariners' bullpen in the eighth and ninth before breaking through in the 10th inning.
The Padres got to Seattle reliever Yoervis Medina (1-1) to win it in the 10th. Denorfia legged out an infield hit and advanced to third on Everth Cabrera's seeing-eye bloop single near second base. Headley was intentionally walked before Venable's game-winner.
"I was really comfortable up there," Venable said. "I got a decent pitch to hit and was able to find a spot where they couldn't make a play. It's nice to win one like that. It shows great resolve all around."
Luke Gregerson (3-2) pitched a scoreless 10th to earn the win.
Padres C Yasmani Grandal didn't shed much light on his use of performance-enhancing drugs on Wednesday in his first news conference with home media since testing positive for elevated levels of testosterone in November. He did say that he took an illegal substance in order to stay healthy, not to sustain an edge over the competition. He didn't divulge further details about PED use, citing an open MLB investigation surrounding the issue. . Mariners OF Michael Morse remained in Seattle when the team left for Wednesday's game at San Diego due to a quadriceps strain suffered on Tuesday. He was examined on Wednesday, and Mariners manager Eric Wedge said he believed Morse's would gone 3-5 days and that he could catch up with the team during a weekend series at Minnesota. . 1B Justin Smoak, out since Saturday with a strained oblique muscle, took batting practice and pinch hit in the eighth inning. . Padres INF Logan Forsythe, on the 60-day disabled list with plantar fasciitis, moved his minor-league rehab assignment from extended spring training to Triple-A Tucson. . Seattle 2B got his first major-league hit in the fifth inning.