A blockbuster of a summer duel has turned into a trilogy between Mahe Drysdale and Robbie Manson to determine who will represent New Zealand in the male single sculls berth at September's rowing world championships in Bulgaria.

Originally the expectation was that Rowing New Zealand would apply standard procedure and select one athlete to represent the class at the trials which follow this week's national championships (duel 1).

Instead - because no-one else wants to contest the berth - they will let both race overseas at the June 21-24 World Cup in Austria (duel 2), and the July 13-15 World Cup in Switzerland (duel 3). The best performer in that final regatta will get the nod.

The scenario contrasts with a previous example in the genre, ahead of the 2008 Beijing Olympics. Drysdale (by then a three-time world champion) was challenged in the trials by Rob Waddell (a three-time world champion and Sydney Olympic gold medallist).

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Their duel required a decider which Drysdale won in front of a packed Lake Karapiro embankment when Waddell suffered a recurrence of the heart condition "atrial fibrillation".

Manson looks the favourite on the basis of early season form, beating Drysdale at the Christmas regatta and North Island club championships. The 28-year-old won both his World Cups last year – and overtook Drysdale's world-best time in the process - but finished fifth at the world championships as he battled back from injury.

Drysdale is returning after a year off following his Games triumph by less than the width of a bow ball in Rio. The 39-year-old's pedigree is undisputed as a double Olympic and five-time world champion. He is training under Calvin Ferguson – his coach from 2007 and 2008 – after the departure of former mentor Dick Tonks from Rowing New Zealand's stable.

Drysdale was happy with how the selection procedure will play out.

"We tend to select our crews early in March, and while that can work in some cases, in others it can be held open.

"Rowing New Zealand have decided early in the cycle they will give us both time, and I think that works pretty well.

"Not having to peak at this time of year is a positive. Being quick a few weeks out from a world champs is a lot easier than doing it now and going back to do it again. We're at the selectors' discretion. This is obviously a big shift in their thinking, but a progressive one."

One priority for Drysdale is dropping weight. He started at "a daunting prospect" of around 120kg, has removed 14kg but wants to shed about 5kg more to get to an optimum 101-102kg.

"I'm pleased with my training, but I'm obviously not where I want to be racing-wise.

"It's frustrating, I have the building blocks in place, now it's just a matter of putting them together at high intensity ratings.

"I'm hoping for a miracle to come out and find speed at high rates in the national champs."

If both make the final, they will race for the national single sculls title next weekend at Lake Karapiro.