Western Kentucky, the 12th seeds in the South Region, led by 17 points before fifth-seeded Illinois turned up the defensive pressure to draw within 74-72, but two free throws by the Hilltoppers in the last second clinched the upset.
Earlier, second-seeded Memphis narrowly avoided a huge shock by rallying to record a 81-70 win over 15th seeds Cal State-Northridge, who led by six points with 10 minutes to play but wilted down the stretch.
Since the NCAA went to the 64-team format in 1985, only two second-seeded teams have lost to a number 15.
Sophomore guard Roburt Sallie, who averaged less than five points during the regular season, led Memphis with 35 points.
"My hat is off to Northridge," said Memphis coach John Calipari. "They were not intimidated. They did not back down.
"They played us like teams in our league play us, which is they're not afraid, they battle, they go at us and they were not going away."
Sixth-seeded UCLA also had a scare before escaping 65-64 against Virginia Commonwealth in the East.
Top seeds North Carolina and Connecticut cruised into the second round. North Carolina bounced Radford 101-58, while Connecticut crushed Chattanooga 103-47.
Memphis's reward for getting past the Matadors will be Saturday's second round match up with 10th seeded Maryland, who defeated number seven California 84-71.
"At this time of the year you just try to march on to the next game," said Calipari, whose team lost to Kansas in the title game last year.
"I'll watch this tape when I get back at the hotel and try to figure out what in the world did they do to us.
"They had a great game plan ... and we had to have a guy get 35 for us to win the game."
The tournament, which has massive appeal in the United States, concludes when the national champions are crowned in Detroit on April 6.
