Nunn, rated No. 56 in the ESPN 100, committed while he was on an official visit to Illinois on Saturday. Groce made an in-home visit last Sunday.
"It was to the point where nothing else was going to get better than here," Nunn's father, Melvin, said Saturday. "There's a lot of history at Illinois to do with it. That's what it's all about. He wants to continue the history. He did it at Simeon and now wants to continue it at the state school and be a part of a national championship at his state school.
"I think Kendrick believes he can get this rolling. He wanted to be a part of something he got started. Groce and his coaching staff want him to be a part of something he got started."
Nunn, a 6-foot-2 shooting guard, is considered the state of Illinois' second-best senior, behind his high school teammate, Jabari Parker, the nation's top-ranked player. Nunn averaged 15 points last season for Simeon, which won its third consecutive state championship. He also helped Team USA win gold in the FIBA U17 World Championship this summer.
"It means for Illinois now they have secured a commitment from a high-level player not only from the state of Illinois, but the city of Chicago," ESPN recruiting coordinator Reggie Rankin said. "I think that possibly can open doors for future players. I think it's a great fit for Nunn because he'll be terrific in Groce's style of play. He'll be able to attack on the break, slash in the halfcourt and play on and away from ball screens. I think it's just a win-win situation for both of them."