Jamie Vardy scored after 21 seconds to set Leicester City up for a victory at Southampton that sent them back to the top of the Championship.
Former England striker Vardy traded passes with Stephy Mavididi before stunning all at St Mary's with his cool side-foot finish to give the Foxes the ideal start.
Kasey McAteer met a fine Kiernan Dewsbury-Hall through ball to make it two after 18 minutes and, while a defensive error gifted Sam Edozie a Saints response, Wilfred Ndidi put Leicester 3-1 up in first-half injury time.
The excellent Mavididi ran from deep inside his own half to add a fourth with a superb solo goal against a side that beat the Foxes twice last season in a Premier League campaign that ended with both being relegated.