Zachery Peacock's two free throws pulled Georgia Tech even for the first time at 69-all with 32 seconds left in regulation.
Georgia Tech's Gani Lawal blocked a shot by James Florence, who had 26 points, with 4 seconds left in regulation to force the overtime.
A 16-0 run gave Mercer a 58-40 lead with 12 minutes left in regulation. Georgia Tech (2-0) used its pressure defense to rally.
E.J. Kusnyer had 22 points for Mercer (3-1), already had road wins at Alabama and Auburn of the Southeastern Conference.
Lawal led Georgia Tech with 27 points, and Peacock added 19.
SOUTH CAROLINA 73,
USC UPSTATE 53
In Columbia, Zam Fredrick scored 23 points to help South Carolina roll.
Fredrick hit three of the Gamecocks' (3-0) eight 3-pointers. Devan Downey scored 16 points, and Evaldas Baniulis had 13 -- all in the first half.
First-year coach Darrin Horn became the first South Carolina coach to win his first three games since Johnnie McMillian started 5-0 in 1944 in his lone season.
South Carolina used a 22-5 run to take a 35-12 lead with just under 7 minutes left in the first half.
For the Gamecocks, Butler High alum Austin Steed had seven points, and Josey High grad Dominique Archie scored four.
CLEMSON 71, CHARLOTTE 70
In Charlotte, N.C., K.C. Rivers scored a go-ahead layup with 43.2 seconds left to lift Clemson over Charlotte.
Rivers scored 12 points and pulled down eight rebounds for the Tigers (5-0), who led by as many as eight points early in the second half. Demontez Stitt scored 14 points for Clemson.
Clemson pulled ahead 50-42 with less than 15 minutes to go, but the 49ers (2-3) pulled even at 67 with 1:01 remaining.
Rivers' layup with 43.2 seconds left put the Tigers ahead 69-67.
Clemson then pulled ahead by four on a pair of Stitt free throws with 16.4 seconds left, and Lamont Mack's 3-pointer 10 seconds later cut Clemson's margin to one.