Taking on the longest event at the Tokyo Games just six days after completing his 11th Tour de France, the Australian team leader dropped out of contention as the attacks began inside the last 50km.

One of 48 riders backing up from the Tour, Porte had the consolation of seeing a pro teammate claim victory for the second Olympics running.

Five years after BMC Racing's Greg Van Avermaet, of Belgium, won after Porte crashed out in Rio de Janeiro, his INEOS Grenadiers colleague Richard Carapaz added Ecuador's second Olympic gold medal to last week's Tour de France podium finish.

Belgian Wout van Aert won the silver medal and Slovenia's Tadej Pogacar added a bronze to his back-to-back Tour crowns.

On a course featuring 4865m of elevation and taking in the lower slopes of Mount Fuji, Porte finished in 48th place, 10 minutes 12 seconds behind Carapaz, with compatriots Lucas Hamilton and Luke Durbridge 71st and 72nd respectively.

Hoping to become just the second Aussie to claim a medal in the event after Victorian Clyde Sefton's silver in Munich in 1972, Launceston's 36-year-old father-of-two had received some strong backing from Athens Olympic track champion Stuart O'Grady.

"Richie's a Tour de France legend and one of the greatest cyclists to come out of Australia," O'Grady told Channel Seven.

On Wednesday Porte and South Australia's two-time world champ Rohan Dennis will also represent Australia in the individual time trial over two laps of a 22.1km course featuring a total of 846m elevation.

The race will mark the end of a stellar 2021 which saw Porte podium in three week-long tours.