But in Game 2 of South Pontotoc's Class 3A semifinal series against Nettleton, the senior turned the tables.
Costilow scored what was eventually the deciding run on her two-out, two-run home run in the top of the seventh inning in the Lady Cougars' 5-4 victory that eliminated the defending 3A champion Lady Tigers.
South Pontotoc, 1-0 winners in Game 1, advances to their first state championship series in program history.
The Lady Cougars will square off against Southeast Lauderdale beginning at 12:30 p.m. Saturday at Ridgeland's Freedom Ridge Park.
"I take a lot of responsibility for the pitching, and when they hit one over like that it does a lot to me," said Costilow. "So I felt like I was taking up the slack from where I let them down."
Her shot put the Lady Cougars up 5-2, but Nettleton (26-4) cut their deficit back to a single run in the bottom half on Jabria Pounds's second homer of the game.
The Lady Tigers put the tying and winning runs on base with one out thanks to a dropped line drive and Lauren Baldwyn's fourth hit of the contest, but consecutive fly outs sealed the win for South Pontotoc.
Breanna Kitchens caught the final pop up in left-center for her 14th putout of the day as the Lady Cougars (23-11) capitalized on fly ball outs throughout the series.
"We hit the ball hard, but it was right at them," said Nettleton coach Dana Rhea. "They outplayed us tonight and got the hits when they needed them and we didn't."
Nettleton held a 2-1 lead heading to the sixth inning before Kaitlyn Cox's RBI double put South Pontotoc up 3-2. Cox was 5 for 6 in the series.
A crisply played Game 1 ended with a walk-off in the seventh as Breauna Holley scored on a sacrifice fly by Katelyn Ferguson.
"I thought the first win was big," South Pontotoc coach Adam Patterson said. "My girls kept fighting and played hard."