Shields allowed three hits in seven scoreless innings and was part of a combined four-hit shutout in Kansas City's 3-0 win over the Detroit Tigers on Friday night to give the Royals a sweep of a day-night doubleheader.
Kansas City won the first game 2-1 as Duffy took a no-hitter into the sixth inning and pitched a combined two-hitter.
"Huge day for us. Shields came back and, basically, matched the outing that Duffy had," said Eric Hosmer, who homered in each game.
Royals pitching held the Tigers to a run and six hits combined in the two games.
"You play 18 innings against a team like that and give up one run, you're doing something special," Kansas City manager Ned Yost said.
Tigers manager Jim Leyland said he isn't worried about his AL Central-leading club.
"It's not a concern. We got what we got. It's pretty simple," he said. "Both teams got great pitching, and we didn't must enough offense."
The Royals gained two games on Detroit to move within 6½ games of the Tigers.
"We wanted to come in here and make a statement against these guys," Yost said.
Shields (8-8) won his fourth game in his past five starts. He walked four and struck out one.
"Tonight I was pounding the strike zone but just missing with some of my pitches," Shields said. " ... I've been pitching well all season. I've got a lot of quality starts. That's what I care about, not wins and losses."
The Tigers threatened against Shields in the second and third innings.
But Shields retired the next three hitters, using mostly his change-up.
Miguel Cabrera was retired on weak roller to Shields between the mound and first base on which the runners moved up to second and third.
Shields then got Victor Martinez to line out to right to end the inning.
"It's huge, especially with their lineup and who was coming up," Shields said. "You've got Miggy, Fielder and Victor, whose one of the hottest hitters in the league ... and Miggy, the best hitter in the league."
Jose Alvarez (1-3), called up from Triple-A Toledo to make the start, took the loss. He gave up two runs and five hits in 5 2/3 innings, walking three and striking out five.
Alvarez's only big mistakes came in the third with two outs. He walked Jamey Carroll and Hosmer followed with his second home run of the day, that gave the Royals a 2-0 lead. Hosmer blasted Alvarez's 1-1 change-up deep into the right-field stands for his 14th homer.
"I felt good, but I made one big mistake," Alvarez said. "I'm here to win games for this team, and I didn't do that, so I'm disappointed."
Alvarez was replaced by Jeremy Bonderman with two outs in the sixth. Bonderman got the final out of that inning and the first two outs of the seventh before being replaced by Phil Coke and Al Alburquerque.
Carroll's sacrifice fly in the seventh made it 3-0. It was an unearned run off Bonderman, whose throwing error on an attempted pickoff throw sent Chris Getz from first to third. He scored on the sac fly.
Duffy (1-0) beat Justin Verlander in the opener.
Duffy was called up from Triple-A Omaha to make his second big league start of the season, and didn't allow a hit until Cabrera's hard-hit, two out single in the sixth.
"I was glad that they finally got one, honestly, because I just wanted to pitch," said Duffy, who underwent Tommy John surgery. "I wanted to protect this arm."
Kelvin Herrera replaced Duffy in the seventh and retired the side in order. Aaron Crow worked the eighth and allowed Santiago's pinch-hit home run leading off the inning. Crow got the next three batters.
"Would have liked to have been able to accomplish two things, win and keep the bullpen fresh," Verlander said. "Just didn't happen that way. Felt like I threw the ball well. Just their guys pitched extremely well, which is they've done all year."
Verlander (12-9) allowed two runs and seven hits in eight innings. He struck out five and didn't walk a batter.
"Our pitching was terrific tonight, both of them," Leyland said. "I'm certainly not going to nitpick what they did."
NOTES: With two outs in the second inning of the first game, Bonafacio hit a soft liner that Detroit shortstop Jose Iglesias reached to his right to catch on the backhand. The ball popped out of his glove back over Iglesias head, but he twisted his body to catch it for the inning's final out. ... In the eighth, Bonifacio, playing center field, and right fielder David Lough collided after Bonifacio made a running catch of Cabrera's long drive on the warning track in deep right-center. ... It was the Royals' first doubleheader sweep in Detroit since Aug. 5, 1984. ... Royals manager Ned Yost said third baseman Mike Moustakas, who has been out since leaving Monday's game with a tight calf, would do some work on the field on Friday to assess his progress. ... The Royals' Billy Butler continued his mastery of Verlander in the first game. Butler was 2 for 3 to run his career mark against the Tigers' star to 27-for-62 (. 435). ... Saturday's scheduled starters are Wade Davis (6-9, 5.29 ERA) for Kansas City against Doug Fister (10-6. 3.60) for Detroit.