When an attack on the final climb in the closing kilometres threatened his lead, the BMC leader dug deep and reeled back stage winner Mattia Cattaneo (Androni Giocattoli-Sidermec) to save his victory by two seconds over the Italian rider.
"I am really happy with the form I have shown here, and of course, to take the win," Dennis said. "Normally I am not in the best condition at the early races in Europe, so this race has definitely been a big confidence boost for what's to come over the rest of the season."
The 171.4km final stage from Aix-en-Provence to Marseille started with a three-rider breakaway that provided the impetus for the BMC-led chase for most of the day. BMC's Tom Bohli did the lion's share of the work, eventually reeling in the escaping trio with about 20km left to race.
Attacks from all sides quickly ensued, with Cattaneo launching his successful late challenge on the final climb into Marseille.
BMC's Brent Bookwalter went off in pursuit with two other riders, but when Cattaneo's gap extended to 25 seconds, it was up to Dennis to take control and finish brining him back. By the line, Dennis had clawed back all but two seconds from the stage winner.
"I was a little bit nervous when Cattaneo attacked," he said. "I wasn't totally sure we could bring him back, and when he was still 14 seconds ahead with 1.5km to go, I knew it wasn't going to be easy.
"When I made my move, it was a last-ditch effort really, so I was pleased it worked out. It's been a close race every day and, with only three stages, there isn't any time to waste. It all comes down to who makes the right move at the right time."
Dennis was quick to thank his teammates for the victory.
"It was our race to lose coming into today's stage, but we proved that we had the strongest team in the end,” he said. “We did everything ourselves today to control the race, and we were able to pull off the overall victory. It has been a full team effort right from the start of this race and I wouldn't be in this position if it had just been me."
Sports Director Yvon Ledanois held out special praise for Bohli, who he said rode on the front of the peloton for more than 100km to bring back the breakaway.
"Then once the breakaway was caught, we pulled hard at the front on the final climb, and in the end, Dennis showed that he was motivated to take the win here," Ledanois said. “For me, this is an important win for his confidence and morale before the UCI World Tour races which lie ahead."