Britain's Chris Froome looks up during a press conference ahead of the start of the Tour de France. Phto: Christophe Ena/AP

Tour de France favourites Chris Froome and Richie Porte believe this year's race will keep fans enthralled right to the end.

With a limited number of high mountain summit finishes and only 36.5km of time-trialling, Australian Porte says the other stages will be a "free for all" that could "turn the race upside down" while Froome insisted it will "favour more aggressive riders".

Briton Froome is aiming for a fourth Tour title having won already in 2013, 2015 and last year, while Porte, his former Team Sky domestique, is looking to deliver on his strong pre-Tour form.

Throw in the likes of last year's Vuelta a Espana winner Nairo Quintana, Spanish veteran and two-time former champion Alberto Contador, French hope Romain Bardet and Criterium du Dauphine winner Jakob Fuglsang, and a major battle is expected over the next three weeks.

With 3,540km of racing ahead, starting with Saturday's 14km time-trial around Dusseldorf, much will happen before the Grand Boucle arrives in Paris on July 23, but Froome's Sky boss Dave Brailsford believes no-one can afford to simply sit back and wait to see what others are doing.

"It's pretty obvious that whoever's going to win this race is going to have to be pretty offensive," said Brailsford.

"It's going to be a pretty open and exciting race, one where you've got to be on the front foot. It's not going to be a defensive race, it's going to be an aggressive race."

Three-time Tour de France winner Britain's Chris Froome, far right, ad his teammates stop at a traffic light during a training. Photo: Christophe Ena/AP

Despite having already won three Tours and having become a father since his last success, Froome insists he hasn't lost any of the desire that took him to the top in the first place.

"The hunger hasn't got any less for me, I'm as motivated as ever as I've got so much more to race for now," said Froome, who like Porte is 32.

"This is potentially my fourth Tour de France title I'm here to try to get.

"It's massive, the challenge is even bigger this year. I feel as if the level of my rivals is even higher, on a difficult course as well.

La Mondiale team riders including Jan Bakelants and Romain Bardet in action. Photo: Christian Hartmann/Reuters

"I'm here with all the motivation I had before."

'More than a two-horse race'

Froome and Porte have been taking turns this week to proclaim the other as the race favourite but Porte did admit that he doesn't feel Froome's Sky team is the "steamroller" it once was.

"Sky is probably not as strong as it was in the past, but how it affects their tactics I don't know," he said.

The Australian warned against focussing on a straight battle between him and his old team-mate, though.

"I don't think it's just going to be between Chris and I, there's so many brilliant bike riders here, you can't just focus on two guys," he said.

BMC Racing rider Richie Porte of Australia smiles. Photo: Benoit Tessier/Reuters

"It's more than a two-horse race."

Certainly, Colombian Quintana will be aiming for overall victory having finished second to Froome in 2013 and 2015 and third last year, when Bardet took second.

He's changed his preparation this year by riding in May's Giro d'Italia, believing he performs better in his second Grand Tour of the year than the first.

Contador knows how to win the Tour, although his best years are probably behind him.

His best Tour finish since being stripped of a third win in 2010 for doping was fourth in 2013.

Riders of team AG2R La Mondiale line up for a group picture before a training session. Photo: Wolfgang Rattay/Reuters

Twice in the last three years he's failed to finish due to crashes.

Bardet's entire season revolves around the Tour while Dauphine winner Fuglsang could be a dark horse.

Quintana's Moivistar team-mate Alejandro Valverde has been the stand-out rider this year, winning the tours of Catalonia, Andalusia, Murcia and the Basque Country, as well as the two Ardennes Classics in Liege-Bastogne-Liege and Fleche Wallonne.

He wasn't at his best in the Dauphine but showed he's not lost any of his attacking flair as he launched a daring bid for victory on the final stage climb to the finish alongside Fabio Aru, the 2015 Vuelta winner from Italy and another strong outsider.