By 10pm on Sunday, rowing fans will know who will be New Zealand's men's single sculling representative at September's world championships in Bulgaria.

Mahe Drysdale and Robbie Manson are duelling for the spot in a winner-takes-all scenario at the World Cup in Lucerne, starting with tonight's heats.

Former Olympic and world champion Rob Waddell knows what the pair are facing.

He lined up against Drysdale in a best-of-three trial series to decide the country's Beijing Games candidate in 2008.

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At 1-1, spectators flocked to Lake Karapiro to see who would earn the coveted oars.

Unfortunately Waddell suffered a recurrence of his atrial fibrillation, a heart condition which left him feeling like he was rowing through mud.

Waddell described his mindset leading to the finale.

"The last thing you want to be thinking about is the result. It's inevitable that's where your mind wanders, but I suspect most athletes are trying to think about the process not the outcome.

"You're trying to fool your mind into thinking the right things. It's not about being tough; it's more about being clever."

Drysdale, who recently battled a cold, has overcome plenty of tight races to stand atop the podium since winning his maiden world championship in 2005. The 39-year-old took last season off after his Rio Games triumph by less than the width of a bow ball, but his pedigree is undisputed as a double Olympic and five-time world champion.

However, Manson looks the heir apparent. The 28-year-old's trajectory across the past two seasons make him favourite, barring injury or gear failure. He won both his World Cups last year — overtaking Drysdale's world-best time at Poznan in the process — but finished fifth at the world championships while battling back from injury.

A Kiwi, courtesy of Drysdale and Waddell, has been the best men's single sculler at the world championships or Olympics for 10 of the past 20 years.

Manson looks ready to assume the mantle. He had superior early season form, beating Drysdale at the Christmas regatta and North Island club championships as his rival sought to drop from around 120kg to an optimum 101kg-102kg. Manson then put 25.82s between them in the final of the national championships.

Eric Murray, a double Olympic and six-time world champion in the pair, expects Manson to hold sway.

"Mahe's one of the best single scullers ever and Robbie's coming through with the world-best time.

"Even though he doesn't quite have the results, Robbie has a lot of experience from the double and the quad, and two Olympics under his belt. It was a shame he got injured before the world champs.

"As long as he's on his game, he's been dominant all summer and at the [Austria] World Cup, so my money's on him."

Both athletes claimed there was room to sharpen up before Lucerne. Manson trained in Munich while Drysdale finished second to his old rival and incumbent world champion Ondrej Synek at the Holland-Beker regatta in Amsterdam on July 1.

Lucerne's Rotsee is arguably the world's fairest course to decide Drysdale and Manson's respective fates. It has minimal current, and the surrounding hills shield the wind.