In a strange, storm-infested tournament that doesn't want to end, Seo did just about everything she could to win her first major except control the weather. She shot a pair of 3-under 68s on Sunday to finish at 3-under 281 for a one-shot lead over her South Korean rival, So Yeon Ryu, who had three holes left when darkness halted play.
Cristie Kerr was another shot back with two holes left.
Seo will sleep on the lead - though not as comfortably as she could have after missing a 3-foot par putt on No. 17 - and then has a chance to wake up today, come to the course, never touch a club and walk away with the trophy.
"I can sleep very well, so I don't worry about that," she said.
But she couldn't celebrate quite yet.
Rain delayed play for the fourth time in four days - this time for 2 hours, 37 minutes - and left 28 players still on the course, three of whom are still in range.
There's Ryu, who shot 69 on her first trip around the course Sunday morning and has at least one decent birdie opportunity - the par-5 17th - awaiting this morning.
There's Kerr, a two-time major winner who isn't conceding anything. She was getting ready to do an interview after darkness fell when she saw Seo hugging friends and family.
"The tournament is not decided yet," Kerr said. "I think she's over there celebrating. We all have a chance. I'm going to go out and swing for the fences and hopefully tie it up."