The sixth-ranked Badgers hadn't lost a game, either. They had five wins by 21 points or more and four shutouts. All that remained between them and a massive meeting with No. 4 Ohio State next week in Columbus was an Illinois team that had lost 26 of 30 Big Ten games under coach Lovie Smith, including the first two this year.
All of Wisconsin's positive trends ended Saturday. Illinois' James McCourt hit a 39-yard field goal as time expires for a 24-23 win -- the Badgers' first deficit of the season.
The Illini, who came in as 30.5-point underdogs, became the first team to win outright as a more-than-30-point underdog since Iowa State (plus-30.5) stunned Oklahoma in 2017. According to ESPN Stats & Information research, Illinois' upset marks the largest in a Big Ten Conference game since Northwestern, a 32-point underdog, beat Minnesota in 1982.
"I don't think we played complementary football," Wisconsin coach Paul Chryst said. "Your last two drives end in turnovers, and giving them a chance. It's always hard to win on the road, and you make it that much harder."
Illinois fans rushed the field afterward as House of Pain's "Jump Around" -- a tradition for Wisconsin's home games between the third and fourth quarters -- played in Memorial Stadium. The Illini beat an AP top-10 opponent for the first time since upsetting No. 1 Ohio State in 2007 en route to a Rose Bowl appearance.
"In my time at Illinois, this is our signature win," Smith said. "We saw so much fight, and we learned a lot about our guys. One of the best wins you could possibly have. ... There's nothing like seeing that locker room celebrate."
Illinois running back Reggie Corbin, who had 62 of his 83 rushing yards in the second half, said he didn't even look at the final kick.
"I rushed the field, man. Saw my dad, gave him a hug," Corbin said. "He was the first person I saw. It was perfect."
Playing on the road for the first time since Aug. 30 against South Florida, Wisconsin committed three turnovers, two fewer than it had in the first six games. After a fourth-down stop near midfield gave Wisconsin, leading 23-14, a chance to put away the game, Taylor raced inside the Illinois 20-yard line but fumbled.
Taylor, who had 10 lost fumbles in his first two seasons, had only one fumble (not lost) before Illinois linebacker Jake Hansen jarred the ball loose.
"I'm trying to make something happen," said Taylor, who eclipsed 5,000 career rushing yards on the first play from scrimmage and finished with 132 rush yards and a touchdown on 28 carries. "You're fighting for those yards, and you've got to make sure you have two hands on it. You don't know who is coming around you."
Illinois went 75 yards in just 1:19 and scored on a 29-yard touchdown pass from Brandon Peters to Josh Imatorbhebhe to cut Wisconsin's lead to 23-21. Badgers quarterback Jack Coan, who was 9-of-10 on third-down pass attempts -- mostly targeting tight end Jake Ferguson -- had his pass intercepted by Illinois' Tony Adams near midfield with 2:32 remaining.
Both Ferguson and Coan took blame for the interception.
"It's just a simple corner route. I put that one on myself," said Ferguson, who led Wisconsin with 77 receiving yards and a touchdown on five receptions. "I didn't get on my depth that I should have. It was there all game. I was getting my depth, and the one I don't, ends up being a pick."
Ferguson didn't think Wisconsin players were looking ahead to Ohio State, and Chryst didn't see any trouble signs during a good week of practice, but linebacker Zack Baun referred to Illinois as "kind of a trap game."
"We didn't have the fire we usually have," Baun said. "We need to do a better job of gradually increasing our energy level going into the game."
Wisconsin's defense came in having allowed as many touchdowns (4) as it had scored. The Badgers had allowed only one touchdown of 40 yards or longer, but Illinois scored on a 48-yard Donny Navarro reception and a 43-yard Reggie Corbin rush.
The Illini, who had lost their past four games to Wisconsin by an average of 24.8 points, averaged 7.1 yards per play in the fourth quarter. Wisconsin had allowed 29 points all season before surrendering 24 on Saturday.
"We've just got to finish better," said linebacker Chris Orr, who led Wisconsin with nine tackles, three sacks and a forced fumble. "We've got to finish drives better, finish plays better, finish tackles better, finish through the point of attack on a play.
"We got outplayed, to be honest. Out-executed."
Wisconsin's offense also struggled against an Illinois team that came in allowing 30.7 points per game and 201.8 rush yards per game. The Badgers reached the red zone five times but scored only two touchdowns. They also missed a 37-yard field goal.
"Those are times when you must get in the end zone," said Taylor, who was substituted out during a goal-line sequence early in the fourth quarter on which Illinois stopped Wisconsin. "We've got to make sure guys are in the right position, we've got to make sure guys are hitting the right holes. Those are the times where you have to get in, when you're that close.
"They did a good job making plays."
Wisconsin players didn't downplay the loss -- "It definitely sucks," Ferguson said -- but also expect to respond well at Ohio State. Orr, the Badgers' emotional leader, noted how teams can lose a second game "because their emotions are still out of whack from the week before."
"This didn't end our season," Taylor said. "We still have games left to play. We have to come back with a focus. If you thought that the season ended today, then I don't know what to tell you. You probably won't be playing, because we definitely have a lot more opportunity out there."