It's difficult to imagine anyone beating men's single sculls rowers Mahe Drysdale of New Zealand and the Czech Republic's Ondrej Synek in Rio; the problem is deciding which of them will win gold.

The two have been titanic rivals for more than a decade, with Drysdale, 37, taking gold at London in 2012 to Synek's silver. Two years later Synek, 33, extracted revenge at the World Championships, repeating the feat in the 2015 event.

Eyeing the pair from behind is Britain's Alan Campbell, the 2012 bronze medallist who will need to perform at a peak level to unseat the two.

''I would say he will have to step up big time to challenge both of those (Synek and Drysdale),'' said Britain's team performance director David Tanner. However, he added, ''I wouldn't underestimate Alan''.

Sir Matthew Pinsent, Britain's four-time Olympic gold medal rower, told Reuters that Drysdale and Synek's ''consistent form'' over the past three years probably makes them the two key contenders but he doesn't believe there's a ''runaway favorite''.

''I think there's something about the single scull that they have a camaraderie and a rivalry all at the same time,'' Pinsent said. ''They respect one another.''

The men's single sculls is one of eight rowing events starting on Saturday at the Rodrigo de Freitas Lagoon.

First up are the heats in men's and women's single sculls, double sculls, and quad sculls, as well as men's pair and lightweight four.

New Zealand will defend three gold medal titles: the men's single sculls, double sculls and coxless pair in which Hamish Bond and Eric Murray are unbeaten in 66 successive races since being teamed up after the 2008 Beijing Olympics. They are heavy favourites to win a second Olympic gold.