The game was deadlocked at 1-all into the final minutes of regulation, but South Gwinnett’s Derek Denton headed a ball into the goal with 2:07 left.
The Comets (11-4-1), in the postseason for the first time in nine years, got their first playoff win in more than 20 years.
South Gwinnett had taken the initial lead in the third minute, but Lassiter (16-1) tied it on Trey Luscre’s header less than 2 minutes later.
The Trojans had a 10-2 advantage in shots on goal.
Pope 5, Dunwoody 2: The Greyhounds trailed 2-1 in the second half, only to mount a big rally in the closing minutes of the Class AAAAA state playoff game.
Pope (12-5-1) took the lead with just 10 minutes left in the contest, when Tai Livnat scored. Zach McCown and Josh Ziskend then scored the final two goals.
Dunwoody had 11 shots on goal, but managed just one in the second half.
Pope will host Gainesville in the second round Wednesday.
Harrison 7, Coffee 0: Cole Guertin led the Hoyas with three goals — including one in the first minute of play — and two assists in the first-round Class AAAAAA state playoff win.
Nathan Vicroy added a goal and an assist, while Kennet Sosuhene, Drew Blankenbecler and Brett Mace each scored once.
Kevin McKay and Zach DeFranks manned the goal for Harrison (15-3), which will host Douglas County in a second-round game Tuesday.
Central Gwinnett 2, Walton 1: The Raiders were unable to rally from a two-goal deficit and lost the first-round game of the Class AAAAAA state playoffs.
Central Gwinnett took a 2-0 lead before Haemo Ku scored for Walton (10-6-1). The Raiders were playing a man down when Ku scored, after Walker Nance received his second yellow card.
Allatoona 3, Heritage-Conyers 1: After Suzanne Arafa scored an unassisted goal in the 32nd minute to tie the game at 1-all, the Lady Buccaneers pulled away for the first-round win in the Class AAAAA state playoffs.
Willough Davis scored the final two goals for Allatoona (12-5-1), which will visit Lakeside-DeKalb on Tuesday.
Walton 3, North Gwinnett 1: Suzanne Zenoni set the tone for the Lady Raiders in the Class AAAAAA quarterfinal with her 6-2, 6-3 win at No. 1 singles.
Johanna Keeline also won 6-1, 6-0, while Katie Riley and Jessica Cramer won 7-6, 6-4.
Walton (16-0) advanced to the semifinals and will host either Harrison or Camden County.
Walker 3, Brookstone 2: Audrey Cunningham and Leigh Norsworthy scored the clinching point at No. 1 doubles, and the Lady Wolverines advanced to the semifinal round of the private-school Class A state playoffs.
With the match tied at 2-all, Cunningham and Norsworthy swept to a straight-set victory, winning 6-4, 6-2.
Walker’s other two points were in singles play. Carly Bloom won 6-3, 6-1 at No. 2, while Mia Pattillo won 6-2, 6-4 at No. 3.
Having now won twice on the road in the playoffs, the Lady Wolverines will try to do the same when they travel to face either Darlington or Pace Academy in the semifinals.
Lassiter 21, Riverwood 7: The Trojans led 12-4 at halftime and never let up in the non-area game.
David Olsen led with four goals, followed by a hat trick from Corey John. Jack Fitzharris had eight assists and two goals, Rykard Martin had two goals and an assist, while seven others scored.
Ryan Smith and Parker Styles manned the goal for Lassiter (15-2), which will play in the first round of the state playoffs Wednesday.
Westminster 11, Pope 6: The Greyhounds were unable to rally and lost the Area 2A-AAAAA game to close the regular season.
Pope (7-11, 5-2) only trailed 7-5 at halftime, but it couldn’t get close enough to hand Westminster its first area loss.
Parker Lavelle led the Greyhounds with three goals, while Liam Lynch had two goals and an assist. Brayton Wadella also had a goal, with Tyler Borko contributing an assist.
Pope will be a No. 3 seed when the state playoffs begin Wednesday.