Johns Hopkins. It was the Seawolves' first-ever win over a ranked team and gave them a 5-0 record for the first time.
Petersen's game-winner was her sixth goal of the afternoon and capped a comeback in which the Seawolves rallied from a three-goal deficit and overcame an injury to their No. 1 goalie.
Petersen also set up Janine Hillier's tying goal with 4:19 remaining. It was her third assist. Petersen's nine points tied Stony Brook's program record for the second time in three games.
"I know teams scout me out as a feeder," said Petersen, who has 15 assists this season. "But I want to be more dynamic and go to goal, not just stay back there and feed. I want to dominate all aspects of the game."
Petersen, a junior who played under first-year SBU coach Joe Spallina last year at Adelphi, had four goals in the second half. Hillier added two late goals as Stony Brook erased a 10-8 halftime deficit.
SBU's starting goalie, Frankie Caridi, left with a knee injury 10 minutes into the game with the score tied at 5. Junior Hannah Perruccio was solid in her place, making two huge saves in the second half.
"It was terrifying,'' Perruccio said, "but I was definitely excited about it."
After Perruccio entered the game, Hopkins (3-2) scored five goals and went up 10-7 but managed only four goals in the second half.
"I just kind of let it settle," Perruccio said. "Stopped playing mind games with myself and relaxed a little bit. my confidence skyrocketed. I screamed at myself, which I don't usually do."
Perruccio was helped by her offense, which tried to turn the game into a track meet in the first half before becoming slow and methodical in the second. The strategy worked. Stony Brook outscored the Blue Jays 7-4 after halftime and controlled the time of possession.
Farmingdale's Candace Rossi led Hopkins with three goals and four assists. Stony Brook junior Demmianne Cook had three goals and freshman Michelle Rubino added a goal, two assists and six draw controls.
Stony Brook will have plenty more chances for victories over ranked teams this month, beginning Monday at home against No. 5 Duke. Then it's off to No. 4 Florida and No. 3 Maryland.
In the past, those might be considered automatic losses. No longer.
Last season, when the Seawolves struggled through a four-win campaign, Perruccio said she couldn't have imagined them where they are now.
"It was definitely different," she said. "It was hard to get used to at first. We didn't know what to expect. Now that [Spallina's] here, we wouldn't want it any other way. He's the only one we could think of coaching at this point."