SEATTLE -- With their Opening Day starter back on the mound, the Kansas City Royals seem headed in the right direction.

Danny Duffy, making his first start in more than a month, pitched 5 2/3 effective innings as the Royals defeated the Seattle Mariners 7-3 Tuesday afternoon at Safeco Field.

Duffy (5-4), who had been sidelined since May 29 with a strained right oblique, allowed two runs on five hits. The left-hander struck out four and walked two.

"Duff is such a huge part of what we do in the clubhouse and on the field, it gives us another boost of confidence," Royals third baseman Mike Moustakas said. "I mean, he's our No. 1 guy. It's a lift for the entire team."

Added second baseman Whit Merrifield: "He's one of the elite pitchers in the American League. To have him back in the rotation is big for us."

Merrifield and Moustakas hit home runs for Kansas City (43-40), which won its third game in a row and remained tied with the Minnesota Twins and Tampa Bay Rays for the American League's second wild-card berth.

Felix Hernandez (3-3) took the loss for the Mariners (41-44), who absorbed their sixth consecutive home defeat.

Hernandez went six innings, allowing six runs (five earned) on six hits, including the two home runs. He walked four and struck out five.

"The Royals have got some momentum going, are playing some good ball ... and we're not," Mariners manager Scott Servais said. "I thought we'd put a little more pressure on Duffy, his first game back. We got a couple of runs early, but he settled in."

The fireworks started early, as both teams scored in the first inning.

Merrifield led off the game by hitting Hernandez's first pitch into the Seattle bullpen in left-center field, his seventh homer of the season.

"As a leadoff hitter, every once in a while you've got to swing at the first pitch just to keep it in the scouting report," Merrifield said. "I've done it two or three times this year with no success. It was good to put a barrel on it and watch it go out."

The Mariners responded in the bottom of the inning as Jean Segura and Danny Valencia singled, then advanced to second and third on Robinson Cano's groundout. Designated hitter Nelson Cruz, making his first start since Saturday in Anaheim, when he injured his right knee sliding into second base against the Los Angeles Angels, lined a two-run single off the wall in right-center field to make it 2-1.

"It was probably not the pitch to throw to Cruz right there," Duffy said. "I just wasn't thinking, and he was probably sitting dead red. He's got some thump."

After the shaky first, Duffy allowed just two more hits.

"I felt like I fell right back into place," he said. "I knew if I could put up some zeros after (the first inning), we could win."

Said Royals manager Ned Yost: "He looked great, just like Danny. We were cautiously optimistic knowing how he looked in his rehab starts and simulated games."

The Royals regained the lead in the fourth, as Eric Hosmer led off with a single and Moustakas hit a one-out homer 401 feet midway up the lower deck in right field. The two-run shot, Moustakas' team-leading 24th of the season, also came on the first pitch of the at-bat against Hernandez.

"Against a guy like that, you've got to have a good plan," Moustakas said. "He's been one of the best pitchers in the game for a long time. I was just fortunate to put some good wood on it."

Moustakas homered for the second consecutive game in the series.

"He's amazing to watch, to get on a streak like that, a homer streak," Yost said. "You've just got to ride that as long as you can. He's having a special year."

Brandon Moss followed with a double off the center field wall, moved to third on a groundout by Alcides Escobar and scored when second baseman Cano bobbled a grounder by Alex Gordon for an error. That gave the Royals a 4-2 lead.

Kansas City added two more runs off Hernandez in the fifth. Following walks to Jorge Bonifacio and Hosmer, All-Star Salvador Perez lined a one-out double down the left field line, scoring both runners to make it 6-2.

The Mariners put two runners in scoring position in both the sixth and seventh innings but came away with just one run.

With runners at second and third in the sixth, Peter Moylan relieved Duffy and struck out pinch hitter Ben Gamel to end the inning.

Seattle scored once in the seventh to pull within 6-3.

With one out, Carlos Ruiz walked and pinch hitter Boog Powell hit a grounder to first base. Hosmer tried to get the force at second, but his throw hit Ruiz in the shoulder and ricocheted into left field, allowing the runners to advance to second and third.

Segura grounded a run-scoring single to left field, with Powell held at third. Moylan got out of the jam by getting Valencia to ground into a double play.

The Royals closed the scoring in the ninth, as Bonifacio hit a two-out triple off Yovani Gallardo and scored on a wild pitch. That was the only hit Gallardo allowed in three innings of relief.

NOTES: Kansas City turned three inning-ending double plays. ... Seattle dropped to 3-31 this season when scoring three or fewer runs. ... Mariners SS Jean Segura went 4-for-5 and is batting .650 in his career (13-for-20) on July 4. ... The Royals sent RHP Miguel Almonte back to Triple-A Omaha to make room on the roster for Tuesday's starter, LHP Danny Duffy. Almonte had been recalled between games of a doubleheader Saturday against the Minnesota Twins, pitching a scoreless inning in the nightcap. ... The finale of the three-game series Wednesday will feature left-handers who lead their teams in wins. Royals All-Star Jason Vargas (12-3, 2.22 ERA) will oppose the Mariners' Ariel Miranda (7-4, 3.82).