Coming into Wednesday night, St. Bonaventure hadn't beaten a ranked team since 2002.
At No. 24 Illinois, Andrew Nicholson gave them a shot to break that string.
He had 17 points and eight rebounds and he nailed a jumper that gave the Bonnies a 38-30 lead with just over six minutes left in the game.
But a late run by the Illini, including 12 points in the last four minutes by Brandon Paul, beat the Bonnies, 48-43.
"We had good looks," Bonnies coach Mark Schmidt said, sitting beside a silent and clearly drained Nicholson at the postgame press conference. "It's not like we were playing against some Division III team. They're 9-0 for a reason. I wish we made some more baskets, but we didn't."
Illinois coach Bruce Weber warned his team the past few days to be ready for the Bonnies (3-4) and Nicholson. But Weber said Nicholson was even better than he'd expected.
"He took it to us, there's no doubt," Weber said.
The last Bonnies win over a Top 25 team was in February 2002, 80-79 over a Xavier team also ranked 24th.
Paul finished with 17 points. D.J. Richardson added 11 and Leonard had nine points and eight rebounds.
Both teams played sloppy on offense and struggled to get good shots for much of the night. The Illini (9-0) hit just 31.9 percent of their shots and the Bonnies 35.4.
St. Bonaventure went up 38-30 with 6:18 to play on a long jump shot by Nicholson.
Then, after a 3-pointer by Richardson cut the lead to 39-33, Paul took over.
First he hammered home a dunk with 3:58 to play that cut the Bonnies' edge to 39-35 and revved up what had been a quiet crowd.
"That was big," Richardson said. "Brandon got the dunk, and he got some energy from the bench. That's when the crowd got up and gave us some energy."
After a bucket by Demitrius Conger, Paul turned a steal into a layup and, after he was fouled, a three-point play that made the score 41-38.
His second three-point play tied the game and Leonard's free throws gave Illinois its first lead since late in the first half.
Down 48-43, St. Bonaventure's had one last gasp. Nicholson tried a three and missed, and the rebound fell, fittingly, to Paul.
Nicholson's night ended on a sour note, but for much of the night he was the game's best player. His rebounding was the biggest reason the Bonnies beat Illinois on the boards, 36-26.
"He was mobile, agile," said Weber. "You have to play defense before he gets the ball. He's pretty good."
Illinois came into Wednesday's game with a brand-new ranking thanks to its 82-75 upset Sunday over Gonzaga.
The Bonnies, mostly playing a lineup with no one taller than the 6-foot-9 Nicholson, nullified Illinois' biggest and, usually, best weapon, Leonard. The 7-1 center had two points in the first 20 minutes to go with two rebounds and three turnovers.
Leonard's only points over that span came on a 12-foot jump shot with 3:37 left in the half that gave Illinois a 24-22 lead, their last until his late free throws.
Inside, Leonard was frustrated by Nicholson and forward Da'Quan Cook.
"The job we did on Leonard gave us a shot," Schmidt said.
All of the Illini, in fact, found the going tough inside.
Thirteen of their 21 first-half shots and 15 of their 24 points came from behind the three-point line.
St. Bonaventure, meanwhile, methodically worked the ball inside to Nicholson. He had nine points at the half and, along with a dunk by Conger, keyed a 5-0 run that gave the Bonnies a 22-20 lead with 6:18 left in the half.
While Leonard struggled with the Bonnies inside, the rest of the Illini lacked an answer for a pestering, high-pressure perimeter defense that forced bad shots and turnovers. Illinois had nine of them in the first 20 minutes. The Illini were better in the second half, finishing with 13 total.