The Boise Hawks were the best team in the Northwest League in the second half of the season. They are one game away from seeing that promising season come to an end.

The Yakima Bears defeated Boise 5-4 in the first game of the East Divisional series Monday night in front of 1,311 fans at Memorial Stadium.

The best-of-three series shifts to Yakima for Game 2 on Tuesday night. Game 3 would be played Wednesday night, if necessary.

“It’s rough to take that loss,” Hawks first baseman and designated hitter Rock Shoulders said. “But we just have to go on the road and take two at their place and do what we have to do.”

Shoulders produced the biggest swing of the night Monday, and it resulted in a grand slam that gave Boise a 4-1 lead in the fourth inning. But the Bears battled back — and took advantage of four Hawks errors.

Boise is the youngest team in the league, and the players showed their youth in what was the first professional playoff game for every player except catcher Willson Contreras.

“A lot of nervousness, a lot of emotions,” manager Mark Johnson said. “It’s expected, especially with this group … wanting to do so much. But it’s a good experience for them. They’ve just got to pick their heads up and go to Yakima and win two, which they’re very capable of doing.”

Shoulders’ grand slam chased Yakima ace Blake Perry from the game, seemingly giving Boise a big advantage the rest of the way.

But the Bears clawed back into the contest with three runs in the sixth inning. With Hawks pitcher Jose Arias on the mound, right fielder Joe Loftus hit a double that drove in shortstop Kevin Medrano to make it 4-2. The next run came on a hit batter with the bases loaded, and then Yakima tied it on a passed ball.

The score remained 4-4 until the top of the ninth, when Medrano hit a drive just over the head of left fielder Trey Martin. Yakima’s Danny Poma scored on the play, and the Bears had a 5-4 lead going into the bottom of the ninth inning.

Albert Almora led off the ninth with an infield single. He stole second and found his way to third base on a wild pitch. With one out, Contreras and Martin each had a chance to drive Almora home, but neither was able to deliver the needed clutch hit.

“We’ll think about it a bit on the bus tonight, I’m sure everybody will, but once we get there tomorrow it’s a new day,” Shoulders said. “Everything is all about tomorrow. We’re not hanging our heads. I mean, it’s a tough loss, but we’ll come back out tomorrow ready to play.”

If they aren’t ready to play, they’ll be swept out of the first round of the playoffs for the second time in two years. Johnson said he can remind his players of that, but it’s a matter of them going out and executing.

“There’s only one way for them to get better, and that’s by playing in games like this,” Johnson said. “We’ve got to win. The pressure’s going to be there, but now they know what to expect and maybe they’ll settle down a bit.”

NOTES: Boise’s Stephen Bruno came into the game on a 22-game hitting streak. He went 0-for-3 with a walk and two strikeouts. … Yakima won the regular season series 7-5. … Boise starter Pierce Johnson, the 43rd overall pick in Junes’ draft, did his part to set the tone for the Hawks early in the game. He went three innings, allowed two hits and struck out two batters. The one run that Yakima scored when he was on the mound was an unearned run. … If the Hawks have their way, it will be the last time the Bears play in Yakima. The team is moving to Hillsboro, Ore., next season.

Chris Langrill: 377-6424