Anna Teruya and Caroline Campbell staked No. 1-ranked Tower Hill (2-0) to a two-goal lead in the first half.
Liz DeMatteis scored for No. 4 St. Mark's (1-1-1) off a first-half penalty corner with no time on the clock.
But the Spartans, who lost to the Hillers in last year's state final, could not carry momentum over to the second half. St. Mark's earned one penalty corner in the second half and none over the final 25 1/2 minutes.
Tower Hill's Caitlin Harvey made it 3-1 with 19:11 left.
Just before that, the ball was knocking between the sticks of players from both teams as St. Mark's tried to send out the ball. But Tower Hill defender Chelsea Stuart broke forward and, from near the sideline at midfield, sent the loose ball back into Spartans territory. The Hillers soon earned a corner.
Tower Hill didn't score off that corner, but kept attacking. From a few yards out, Campbell directed the ball to Harvey, who scored to return the Hillers' advantage to two goals.
Stuart said the St. Mark's goal right before halftime got her team's attention.
"That gave us our second wind. We wanted to get the goal back," said Stuart, a sophomore who is one of the Hillers' new starters on defense. "We worked well positionally, and we're starting to understand how each other plays."
DeMatteis was impressed by Tower Hill's second-half surge.
"They came back hard at us," she said. "We tried to keep it as close as we could, but they were better. We'll get there with time. It's definitely a good building step. We definitely see where we stand now."
St. Mark's played without senior Katelyn Falgowski, a two-time All-American and state player of the year in 2004 and '05. She is a member of the U.S. Field Hockey World Cup team playing in Spain from Sept. 27 to Oct. 8. The team is currently training in the Netherlands.