The 27-year-old beat Swiss BMC teammate Stefan Kueng by eight seconds with Austrian Matthias Brandle of Trek nine seconds off the pace.
Giro d'Italia winner and time-trial specialist Tom Dumoulin of the Netherlands was fifth fastest, also at 9sec.
The world champion and fan favourite Peter Sagan was 16sec off the pace while his great rival, Olympic champion Greg Van Avermaet was just 30sec behind the winner.
The next two stages should provide bunch sprints giving Dennis the hope of holding on to yellow until at least Tuesday.
Wednesday through to Sunday sees the Tour roll over Switzerland's toughest mountains before a final day individual time-trial over 28.6km on the deciding day.