The eighth-seeded Buckeyes led by as many as 12 in the first half and trailed by six in the second half, but little of that mattered by the arrival the two-minute mark in the game.
Through 38 minutes, nothing had been decided. Ameryst Alston and Martina Ellerbe -- respectively the team's best player and a senior unready to hang up her sneakers for the last time -- changed that with a trio of crucial plays that tipped the game in Ohio State's favor.
Alston, the All-Big Ten sophomore point guard, went first with a driving scoop shot that broke a 73-all tie with 1:53 left.
The Wildcats rushed the ball up the floor, but Ellerbe cut off a drive by Karly Roser, who lost the ball to Ohio State's Cait Craft.
With a lead to protect, the Buckeyes slowed the tempo, and Ellerbe found herself open on the wing against the Northwestern zone. She buried a three-pointer -- her fourth of the game -- to suddenly put Ohio State on top by five, 78-73, with 1:24 left.
From there, the ninth-seeded Wildcats were in scramble mode, and they would not be able to catch up.
The Buckeyes went 8 for 8 from the free-throw line in the last 51 seconds to close out the 86-77 victory and earn a shot at No. 1 seed Penn State in the quarterfinals Friday back at Banker's Life Fieldhouse.
Ellerbe's defensive play was not only pivotal, it resulted in the first defensive stop for either team in a span of nearly three minutes. They traded baskets on a combined 11 consecutive possessions before the steal, and the Buckeyes outscored the Wildcats 8-4 thereafter.
Alston had four of the free throws in the final minute and finished the game with 30 points. A week after suffering a painful shoulder injury in a loss at Iowa, she played 38 minutes and went 4 for 9 from the three-point line.
Ellerbe and fellow senior forward Darryce Moore both posted double-doubles for Ohio State, which improved to 16-17 on the season. Ellerbe had 16 points and a career-high 13 rebounds while Moore added 19 points and 10 boards.
Half of Moore's rebounds came on the offensive end, and she helped Ohio State log a 21-16 advantage in second-chance points.
Maggie Lyon led Northwestern with 23 points while Nia Coffey had 17, Lauren Douglas 15 and Roser 11.
The Buckeyes and Lady Lions are set to square off at noon Friday in a game that will be broadcast by the Big Ten Network. Penn State won at least a share of the Big Ten regular season title for the third straight season and swept the season series against Ohio State.
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