Rays center fielder Desmond Jennings dropped a deep drive by Kyle Seager in the bottom of the eighth for a two-base error and Kendrys Morales drew a walk to set up Smoak's game-winner, which bounced out of the glove of diving right fielder Wil Myers.
"I was just praying it would get down, and luckily it hit some grass out there," Smoak said after delivering his first game-winning hit in the seventh inning or later this season. "It feels good, especially in a 4-4 game with guys on base. One swing of the bat away from winning the game, so it's always a good thing."
Danny Farquhar finished off the ninth for his 13th save as Seattle snapped a six-game losing streak at home while improving to 64-77. The Rays, trying to hang on to the American League's second Wild-Card berth, fell to 77-63.
Ibanez continued emerging from a midseason slump and now has 27 home runs on the season, the third-most ever by a player aged 42 or older behind only Ted Williams (29 in 1960 at 41) and Barry Bonds (28 in 2007 at 42).
Ibanez hit 24 home runs before the All-Star break, but had just one since July 12 until he broke loose with a ninth-inning, pinch-hit pop on Thursday in Kansas City. He followed that with a fourth-inning blast Friday that gave the Mariners a 4-2 lead.
"Another big home run, getting on top of that fastball and hitting it a long way," said manager Eric Wedge. "Maybe he's starting to heat up a little bit, which would be huge for us."
Capps wound up with the victory despite giving up one run on two hits and a pair of walks in a shaky eighth. At one point, Wedge and trainer Rick Griffin went out to the mound because the big right-hander looked so out of sync, but he stayed in and got Evan Longoria to ground out with the bases loaded on a nice backhanded stop by third baseman Kyle Seager.
"I couldn't get comfortable out there, couldn't slow the heart rate down," Capps said. "I was over-amped, I guess. I don't know. It was kind of a [crummy] outing, but Smoak did a great job picking me up and [Mike] Zunino did a great job blocking all those balls because I know I couldn't have been easy to catch tonight. I just couldn't get in a groove out there. That was all it was. My arm felt fine."
Capps helped himself tremendously by picking pinch-runner Sean Rodriguez off second base with two on and nobody out.
"I just got too far away from the bag," said Rodriguez. "I just messed up. Plain and simple."
Shortstop Brad Miller called for the pickoff move, knowing Capps has an excellent one and that Rodriguez was sneaking ahead, anticipating a bunt.
"I know he has a really good move and likes doing that," Miller said. "If it works, it works. If it doesn't, at least we're keeping him close. The guy is bunting, so I'm there anyway. I'm trying to keep him real close. From my angle, I have a good view and I'd have done the same thing [as Rodriguez]. It's tough, because if I'm running, I'm taking a secondary, too, because he holds it and sells it so well. That was pretty big. He always wants to do it, so that was perfect. That was a big out."
Hisashi Iwakuma allowed three runs and eight hits over 5 2/3 innings before leaving with a 4-3 lead. With the no-decision, he remained 12-6 with a 2.97 ERA and continues building his case as one of the American League's premier starters, along with teammate Felix Hernandez (12-9, 3.01).
Wedge lifted Iwakuma with two-on and two-out in the sixth after he'd given up an RBI single to Jose Molina that cut the margin to one run. Yoervis Medina came on to end that inning and preserve Iwakuma's lead after the Japanese right-hander had thrown 98 pitches.
Wedge is being careful with Iwakuma down the stretch as he's now at 196 2/3 innings with the potential of three starts remaining. The 32-year-old never threw more than 201 2/3 innings in a season in Japan and totaled 125 1/3 last year, his first in the Majors, when he spent the first half of the year pitching out of the bullpen.
"I said before we started Spring Training, one of my goals was to stay healthy through the long course of the season and stay in the rotation the whole year," Iwakuma said through translator Antony Suzuki. "We're coming very close to that and that's a good sign for me. I just need to continue pitching my game and keep going forward until the last game of the season."
Seager picked up an RBI in the first on a sacrifice fly and also had an RBI single in the third when both he and Franklin Gutierrez drove in runs with back-to-back base hits. Both hits came with runners in scoring position as did Smoak's game winner as Seattle was 3-for-7 in that situation after going 5-for-50 with runners in scoring position over the previous six games.