Simon Pagenaud has third finish of 13th place or worse in four races, but still has 76-point lead.
Will Power picked up his second consecutive win of the season, cruising to victory in IndyCar's return to Road America.
Dixon's championship hopes take another hit after an engine issue early on at Road America.
Hamilton and Rosberg better than ever at Mercedes.
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Same weekend confirmed for Road America for 2017.
The Verizon IndyCar Series makes its triumphant return to Road America today, and is live on NBCSN at 12:30 p.m. ET.