Ussher (32) finished third in Europe's most popular long-distance triathlon last year but is still considered an "under the radar" quality, with the focus in the men's race mostly on defending champion Michael Gohner, of Germany.
Austrian Normann Stadler is also receiving plenty of media attention during the build-up to the race, after he set a cycle course record at Roth last year (4hr 14min 42sec), to finish fourth.
Organisers say the race will feature the strongest field of males to have competed at Challenge Roth, now in its ninth year.
Ussher, from Nelson, began preparing for the event in May in New Zealand but he had mainly been doing adventure racing since the Challenge Wanaka.
"I was in China before the build-up to this and then did the 24-hour adventure racing championships and then flew to Finland," Ussher said.
"I have been in Finland for the last five weeks with [wife] Elina.
"We [Team Thule] raced a five-day staged race in the north of France.
"That was pretty good but we were stumped by the French rules and interpretation.
"It was hard and French was the official language ... we just misinterpreted some of the rules. But we ended up second."
His recent weeks in Finland have helped him acclimatise to the heat.
Germany is in the grip of a heatwave and ,unusually for Finland, that country has also had temperatures above 30degC.
The men's field at the Challenge Roth also includes New Zealander Keiran Doe, who finished third at the Taupo Ironman, and 2009 Challenge Wanaka winner Chris McDonald, of Australia.
The women's field includes world champion Chrissie Wellington (Great Britain) and top Australian athletes Rebekah Keats and Belinda Granger.
There are 2800 individual starters and 550 teams taking part.
New Zealand entries comprise two teams of three and eight individuals.
