Shields (3-1) gave up one run, struck out five and walked three in 7 1/3 innings, improving to 3-0 with a 1.12 ERA in his last five starts. Even more impressive, the right-hander coped brilliantly with a steady rain that lasted for much of the game.
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Even with lightning crackling overhead, the only break in the action occurred when the grounds crew applied drying agent to the mound and a sloppy infield.
"It was raining pretty hard out there," Shields said. "Having to deal with the delays out there, with that team and that lineup over there, to be able to hold them in check is pretty nice."
So was working with a three-run cushion.
Guyer became the first player in Tampa Bay history to homer during his first plate appearance in his major league debut. Recalled from Triple-A Durham earlier in the day, Guyer drove a 3-1 pitch from rookie Zach Britton (5-2) into the left-field seats with a runner on to make it 3-0 in the second inning.
That represented a week's worth of runs for Shields, who had a run-support average of 1.75 per nine innings -- lowest in the AL.
"These guys gave me a 3-0 lead after the second inning and my job was to keep them in check right there," Shields said. "I think that might have been one of the first times I've had that this year. It's really nice. You're able to be aggressive as a pitcher and attack the hitters more."
Guyer was even more excited.
"I couldn't have scripted it any better," he said. "Definitely exciting. Pretty surreal right now."
When he got to the dugout, Guyer found his teammates sitting stoically on the bench. After an appropriate delay, the players got up and smothered the rookie with hugs and high-fives.
"I was so ecstatic I just wanted to make sure I didn't trip," he said. "It's the last thing I thought would have happened. For me to get a home run, it's pretty cool."
Johnny Damon also connected for the Rays, who are 17-6 since a 1-8 start that included three straight losses to Baltimore to open the season.
In his previous three games against the Orioles, Shields was 0-3 with a 6.48 ERA. This time, he was nearly flawless -- except for a sixth-inning homer by Derrek Lee.
"That's what he's been, he's really been sharp," Tampa Bay manager Joe Maddon said. "Sharp on both sides of the plate, sharp with all his pitches. He's been very frugal with his pitches, getting deep into the game."
Unlike Britton, who went 5 1/3 innings before being pulled after throwing 95 pitches. It was the shortest of his six career starts. He gave up three runs and four hits -- all in the second.
"I don't think it's a good outing, any way you look at it," the left-hander said. "My job was to go deep in the game; I didn't do that at all."
In his previous six starts, Britton allowed a total of one run in the first two innings.
Baltimore used consecutive two-out doubles by Guerrero and Scott to get a run in the bottom half.
The Orioles finished with five hits.
"I am concerned that we're not playing to our capabilities offensively," manager Buck Showalter said. "I have a lot of confidence that that will change [but] we didn't put up much resistance tonight."
- The Rays put RHP Jeff Niemann (back strain) on the 15-day disabled list, which led to the recall of Guyer.
- Scott will have an MRI on his right shoulder Monday to determine the source of lingering pain.
- Tampa Bay is 6-1 this year in games started by Shields.
- Guyer went 1 for 3 with two strikeouts before leaving for a pinch hitter in the eighth.