Slovenia (2-1) took the lead for good on Miha Zupan's 3-pointer with 6:33 remaining. Samo Udrih followed with a layup and Lakovic made a 3-pointer, making it 77-71 with 5:04 remaining.
The Slovenians never let Croatia (1-2) get closer than four from there, delighting their green-clad fans who chanted and danced throughout much of the second half.
Every game in the first two days of Group B action had been decided by double digits.
Roko Ukic scored 20 points, and Marko Popovic and Marko Tomas added 17 apiece for Croatia, which shot 55 percent from 3-point range but only 15 of 26 from the foul line.