CHAMPAIGN—Mike Tisdale had a career-high 31 points and grabbed 11 rebounds and Illinois spoiled No. 25 Northwestern’s return to the rankings after 40 years with an 89-83 overtime victory Wednesday night in the Big Ten opener for both teams.Fellow big man Mike Davis had 20 points and 17 rebounds — his conference-leading seventh double-double — for the Illini (9-4), who beat Northwestern for the 11th consecutive time.John Shurna had 27 points, including four 3-pointers, for the Wildcats (10-2), who moved into the rankings on Monday for the first time since January 1969 when the poll was still a Top 20.The 7-1 Tisdale and Davis combined for 10 of Illinois’ 15 points in the overtime.