The cleanup hitter followed consecutive RBI singles by Alex McClure (Royals) and Cardinals prospects Mike O'Neill and Kolten Wong with his fourth fall homer, a blast to right field that ended a rough outing for Santiago (1-2).
"I'd faced him the inning before and he had thrown me a few changeups in hitter's counts. I got to 3-0 and he threw me a fastball, so I figured he might go back to the changeup," McGuiness said. "I was able to find something up, I put a good swing on it and, luckily, it went out to right."
The former 13th-round pick went 3-for-4 and extended his RBI streak to six games. He's driven in eight runs during that stretch, going 10-for-22 to raise his AFL average to .333.
"I feel good in the box," McGuiness said. "I'm working with the same approach throughout the year, and some days you show up and it works and some days it doesn't. And you've got to stick with it, don't get away from it. I'm getting into good hitter's counts and when they make a mistake, I'm not missing it. Today I was able to handle the ones they left up and got in some runs."
Drafted by the Red Sox in 2009, the Citadel product was traded to Texas in July 2010 in the deal involving catcher Jarrod Saltalamacchia. McGuiness batted .268 with 23 homers and 77 RBIs in 123 games at Double-A Frisco in his fourth Minor League season.
In Arizona, the South Carolina native is focusing on maintaining a consistent, balanced approach at the plate.
"Recently, when I get in trouble, it's when I'm trying to get too big," he said. "I need to stay grounded and stay on my legs and drive the ball up the middle. When I get pull-happy is when I start having trouble."
Surprise (13-10) is in second place in the West Division, so winning has been a nice bonus for McGuiness, who's coming off his first full season at Double-A.
"I have no complaints," said McGuiness, who reached the Texas League Championship Series with the RoughRiders. "We have a great group of guys and it helps when you win. We have have a good staff, a good manager, they let us go out there and have fun, so it's been a good experience overall."
McGuiness missed a lot of time in 2011, appearing in 55 games due to oblique and knee injuries. But he bounced back following a promotion last spring and is keeping that momentum going with Surprise.
"I just feel fortunate to have had the year I had," he said. "The big thing is staying healthy. The year before, I was hurt; it's hard to compete against guys at the top of their game when you're not. But I feel healthy. It was one of those years I was happy to have. I'm just trying to move forward and take it into next year."
Cardinals lefty Kevin Siegrist (2-1) started for Surprise and allowed one hit over three innings. The 2008 41st-round Draft pick threw 39 of 72 pitches for strikes, working around three walks, and is unscored upon in four of five Fall League outings.
Surprise took the lead in the third on an RBI single by Red Sox prospect Bryce Brentz, who exited for a pinch-runner.
Santiago took the loss after surrendering seven runs on eight hits in 1 2/3 innings.