By Gabe Souza, Globe Correspondent

When Chelsea cross-country coach Don Fay saw his sixth and final runner trailing his Mystic Valley opponent with just 100 meters to go in Thursday’s meet, he feared his team’s undefeated status might be in jeopardy.

Knotted at 28-28, Mystic Valley’s Pat Kinnon held a slim lead over the Red Devils’ Hector Mendoza as the race neared its close.

But in a last gasp effort, Mendoza found enough energy to overcome Kinnon in the final stretch and gain a victory for his team with a three-second margin.

“Hector was behind most of the race,” said Fay about his senior runner who set a personal best by 50 seconds. “I was pretty surprised when he came out on top. But he saved the meet for us.”

The Chelsea boys team improved to 4-0 overall while the girls went to 5-0.

Both teams’ undefeated records are a sign of a steadily improving program, Fay said.

“This is the best we’ve had in my four years here,” Fay said. “We’ve got good team chemistry and a lot of stellar athletes.”

Depth in positions is also a strength for Chelsea, Fay said, evidenced by the victory delivered from the No. 6 runner. He believes it could carry his team all the way to a Commonwealth Athletic Conference championship.

“We have eight or nine runners that consistently place well,” Fay said. “That helped us. I thought this would be a close meet. I didn’t think it would be this close, but I’m happy with the outcome.”

Chelsea’s next meet will be at Minuteman Regional High School, Tuesday, Oct. 12. But the matchup Fay and his Red Devils are most looking forward to will be on Oct. 20 when team clashes with defending conference champion Shawsheen Valley Technical.

“They beat us 35-22 last year,” Fay said. “It will definitely be a lost closer this time around.”