SEATTLE -- Brett Gardner recently called Luis Severino the ace of the New York Yankees' rotation.

The 23-year-old right-hander lived up to the billing Thursday night.

Severino pitched seven scoreless innings, and Gardner hit a solo home run as the Yankees defeated the Seattle Mariners 4-1 in the opener of a four-game series at Safeco Field.

"He's been unbelievable," said Gardner, the Yankees' left fielder. "Not just tonight but last week in Boston and the whole first half.

"It's been fun to watch him grow. He makes it easy on the defense because there's not much action for us. He's only going to continue to get better. He's got a great arm."

In his past two starts, Severino has matched up against the Red Sox's Chris Sale and the Mariners' Felix Hernandez.

Severino didn't get a decision Saturday against Sale in a game the Yankees won 4-1 in 16 innings, but he beat Hernandez on Thursday.

"When he's on the mound, we feel comfortable," Gardner said.

Severino (6-4), who didn't pitch in the All-Star Game despite being a member of the American League team, allowed eight hits to the Mariners but got out of several jams. He walked one and struck out six.

"I knew it was going to be tough," Severino said of facing Hernandez. "That's who I compare myself to. Any chance you get to face a guy like that ... it gives me a chance to (get) better."

Seattle's only run came on a two-out double by former Yankee Robinson Cano in the bottom of the ninth inning against Aroldis Chapman.

The Mariners wasted a strong start by Hernandez, stranding 12 runners and going 2-for-14 with runners in scoring position.

Hernandez (5-4) allowed just one run on three hits and two walks in seven innings. He struck out a season-high nine.

"It was an outstanding pitching performance from both sides," Mariners manager Scott Servais said. "Felix was really sharp and gave us everything he had. ... Unfortunately, the other guy was a little better. Severino was about as good as we've seen all year."

Gardner broke a scoreless tie with his homer with one out in the top of the sixth inning. He hit a 2-1 pitch from Hernandez into the right field seats, a shot estimated at 406 feet. It was Gardner's 16th home run of the season.

"He gave me a fastball and kind of left it in the middle," Gardner said. "You don't want to miss a pitch like that."

New York added three unearned runs over the final two innings.

With one out in the eighth, Chase Headley singled to center off left-handed reliever James Pazos, and Gardner reached on an error by Seattle shortstop Jean Segura. Right-hander Tony Zych relieved Pazos and walked Gary Sanchez to load the bases for Aaron Judge.

Judge, who was in a 3-for-28 slump since winning the Home Run Derby in Miami during the All-Star break, hit a run-scoring single to right field to make it 2-0.

Zych got out of the jam by getting Matt Holliday to ground into an inning-ending double play.

The Mariners threatened in the bottom of the eighth against Dellin Betances, as Kyle Seager was hit by a pitch with one out and Danny Valencia grounded a single to right field, putting runners at first and second.

Mitch Haniger hit a comebacker to the mound, with Betances throwing to second baseman Starlin Castro to get a force on Valencia. Castro bobbled the ball, and the Mariners challenged the call by second base umpire Rob Drake, but the out was confirmed after video review.

With runners at first and third, Betances struck out Jarrod Dyson to end the inning.

The Yankees scored twice more in the ninth. With one out, Didi Gregorius and Todd Frazier singled off right-hander Max Povse. After Jacoby Ellsbury flied out to left, Headley hit a ground ball between first and second. Mariners second baseman Cano went into shallow right field to field the ball, but his throw to first was errant, allowing both runners to score.

In the bottom of the ninth, Chapman walked leadoff hitter Mike Zunino and threw a wild pitch before striking out the next two batters. Cano spoiled the shutout bid with run-scoring double to right-center field. Chapman got Nelson Cruz to fly out to right field to end the game.

NOTES: The Mariners acquired RHP David Phelps from Miami on Thursday in exchange for four prospects: CF Brayan Hernandez and RHPs Brandon Miller, Pablo Lopez and Lucas Schiraldi. Phelps was 2-4 with a 3.45 ERA in 44 relief appearances for the Marlins this season. ... First base umpire Tony Randazzo was hit in the head by a line drive from Yankees SS Didi Gregorius in the fifth inning. Mariners 1B Danny Valencia made a diving attempt for the foul ball, and the ball glanced off Randazzo's head, sending the umpire tumbling to the ground. Valencia and Yankees first base coach Tony Pena helped Randazzo to his feet, and Randazzo was checked by Mariners trainer Rick Griffin before play resumed. ... Yankees LHP CC Sabathia (8-3, 3.54 ERA) is scheduled to take on Mariners rookie RHP Andrew Moore (1-1, 5.25) in the second game of the series Friday night.