The Illini (29-1, 15-1) were trying to cap the Big Ten's first unbeaten season in 29 years. Instead, they frittered away a 12-point lead in the second half and didn't score over the final 3 minutes.
Sylvester scored a career-high 25 points - eight more than his previous best - for a team that was banned by its own administrators from the postseason two months ago for alleged NCAA violations.
Ohio State (19-12, 8-8), a team with problems all year hanging onto the ball, didn't have a turnover in the second half. The Illini, who had hurdled tough games in loud environments all season, had seven turnovers after halftime.
It was the second straight season a team coached by Thad Matta ended a run at perfection. Matta's Xavier team handed Saint Joseph's its first loss last season in the Atlantic 10 tournament.
The last team to reach the NCAA tournament without a loss was UNLV in 1991.
The Illini led 64-58 after James Augustine's layup with 3:23 left - but didn't score again.
The Buckeyes ran off the next seven points, with Terence Dials, who had 21 points, scoring on a half-hook before Sylvester drove the lane for a left-handed scoop to cut the lead to 64-62 with 1:40 remaining.
With just over a minute left, Illinois' Deron Williams drove the lane and wrapped a pass around a defender to Roger Powell, but Dials swatted the shot away to teammate Je'Kel Foster. Sylvester missed a 15-footer for the Buckeyes and then Illinois' Luther Head misfired on a 3-pointer with 17 seconds left.
Ohio State called a timeout with 12 seconds left. Foster inbounded to Brandon Fuss-Cheatham who passed to Sylvester on the right wing, in front of the Ohio State bench. His quick shot seemed to surprise the Illini, nestling in the net while a capacity crowd at Value City Arena roared.
After a timeout, a pass was tipped out of bounds in front of the Illinois bench. The Illini had one last chance with 2.2 seconds left. Williams passed to Powell, but his hurried 3-pointer from the top of the circle was well short and off the mark.
Fans rushed the court after the buzzer sounded, snapping photos, jumping up and down and slapping the Buckeyes on the back.
Dee Brown led the Illini with 13 points, although he was rattled all day by chants of "air ball" after several errant second-half shots. He was 3-of-11 from the field and had only five points over the final 34 minutes. Powell and Head each added 12 points.
Williams, who came in averaging 12.8 points a game, was 1-of-7 from the field and finished with two points.
Foster added 10 points for Ohio State, which enters this week's Big Ten tournament as the No. 6 seed.
The Illini were 9-of-27 (33 percent) from the field in the second half.
AP Sports Writer/source U of I office