LATHROP — One down, 13 more stressful matches to go.

Sierra began its Valley Oak League co-championship defense Tuesday with a 4-1 win over Lathrop High at Bennie Gatto Field. While the Timberwolves (1-0, 5-3-1 overall) prevailed by a wide margin, head coach Joe Pires isn’t expecting very many of those results moving forward.

And the league opener wasn’t necessarily stress-free for Sierra. More on that later.

“I told the guys not to get too confident,” Pires said. “Every game is important and it doesn’t matter who we play. The VOL is getting to the point where there are no easy teams — anybody can beat you at any time.”

Defender Abdullah Aslami broke the scoreless tie in the 4th minute following Jesus Coronado’s corner kick. Lathrop goalkeeper Erik De Leon leaped vertically to pull down the ball but could not reel it in, setting up Aslami’s wide-open look from short range.

Standout midfielder Juan Acosta was the table setter for Sierra’s next two goals and they were beauties. In the 34th minute, he and the Timberwolves caught the Lathrop defense on its heels on a free kick. He broke away up the middle and pushed it left to Coronado, who buried it into the opposite panel.

And in the final minute of the first half, Acosta spearheaded another breakaway in which he lofted a through ball to a streaking Henry Diaz, who never broke stride as he finished off his run down the spine of the defense with an uncontested header.

It wasn’t until the 71st minute that Sierra truly put it out of reach. Diaz delivered a corner-kick cross and Bryan Ramos capped the sequence with a head shot.

 “That early goal really helped us out a lot,” Acosta said. “After that it was all us. We did well overall in the first half, but the second half could have been better. That showed us that we still have work to do and we’ll do it on the training grounds.”

It was an uneventful first half for starting keeper Manny Garcia. The same could not be said for back-up Julian Semenza, who was tested in the opening minute of the second half. Mere moments after the kickoff, Semenza turned away a powerful shot from distance and slapped the ball over the crossbar with his fingertips. That would be just the first of his five saves, and that’s not counting several other scary moments for him and the Timberwolves over the final 40 minutes.

“That was the key moment,” Pires said of Semenza’s first save. “If they got that goal in that changes the whole momentum of the game.”

It still sparked the Spartans (0-1, 6-5-1), as they continued to pepper the Sierra goal with attempts. They were outshot 9-4 in the opening half but had an overwhelming 12-4 edge in the second.

Semenza thwarted Estevan Lopez and Cedric Arauzo in the 47th and 48th minutes. In both instances, Semenza bolted off his line and tackled the ball free from each player on fastbreak opportunities outside of the penalty area. Sergio Carrillo had a 1-on-1 from inside the 18-yard line, but his shot was deflected away by Semenza.

The Timberwolves were seconds away from registering the clean sheet, but a questionable handball violation helped Lathrop tally its lone goal. On the final play of the match, Lopez carried the ball toward the right corner and lifted a cross into the box. The ball caromed off the face of Sierra defender Joel Vargas, but he was whistled for a handball.

Vicente Lopez converted the penalty kick, and that’s how the game ended.

“I really wanted the shutout,” Semenza said. “I saw it hit his face. I didn’t think it was a good call.”