Duhamel, from Lively, Ont., and Radford, from Balmertown, Ont., were second with 77.01 points, just behind Olympic bronze medallists Aliona Savchenko and Robin Szolkowy of Germany. The four-time world champs lead at 79.02 and Sochi silver medallists Ksenia Stolbova and Fedor Klimov of Russia were third with 76.15 points.
"It was amazing," said Radford, who won world bronze last year with Duhamel. "We wish we could skate like that all the time. Most of this season, we tried so hard to please everyone and fight for every point. Today we were a lot more relaxed and it's put us in a perfect spot heading into the free skate."
Less than eight points separate the top five pairs. At the Games, Duhamel and Radford helped Canada to the silver medal in the team event and were seventh in pairs.
"We came back from the Olympics feeling like we had achieved all our goals," said Duhamel. "We feel very settled but from this point on we're going to be competing for ourselves and not worry about every little point."
Tatsuki Machida of Japan upstaged compatriot and Olympic gold medallist Yuzuru Hanyu on Wednesday to finish first in the men's short program. Machida, who was fifth at last month's Sochi Games, hit all his jumps and scored a career-high 98.21 points to finish ahead of Javier Fernandez of Spain, who had 96.42 points.
Three-time world champion Patrick Chan of Toronto is skipping the championships. The men's event concludes with the free skate on Friday.
Skating to music from the movie "East of Eden," Machida electrified the capacity crowd of 18,000 at Saitama Super Arena with a near-flawless routine that opened with a quad toe loop, triple toe loop combination. The 24-year-old Machida also included a triple Axel and a triple Lutz in his routine.
"I got motivated when I saw Hanyu win the gold in Sochi," Machida said. "I think this is the best chance I'll have to get a medal and will do my best in the free skate. I was able to get a lot of positive energy from the crowd tonight."
Hanyu fell on the opening quad toe loop and finished with 91.24 points for third place.
"Needless to say the toe loop wasn't good," Hanyu said. "But how I jumped wasn't that bad so I am going to adjust for the free program."
Fernandez, who was fourth in Sochi, impressed the judges with a dynamic routine and got high marks for a triple Lutz, triple toe loop combination.
"One skater did better than me but I am still happy with my performance," Fernandez said. "Sometimes it's hard to get motivated right after the Olympics by my coaches told me to work hard and I can see that hard works pays off."
Tomas Verner of the Czech Republic was fourth, followed by Han Yan of China. Kevin Reynolds of Coquitlam, B.C., was 15th, world junior champion Nam Nguyen of Toronto was 16th and Elladj Balde of Pierrefonds, Que., was 22nd.
Kirsten Moore-Towers of St. Catharines, Ont., and Dylan Moscovitch of Toronto were sixth in the pairs while Paige Lawrence of Kennedy, Sask., and Rudi Swiegers of Kipling, Sask., were 12th.
In Sochi, Moore-Towers and Moscovitch contributed to Canada's team silver and were fifth in the pairs.
"It was nice to skate the program clean one last time. All those run-throughs paid off," Moore-Towers said.
"I felt great, calm and confident," added Moscovitch. "The skate felt good. Our goal is to skate to clean programs and we are halfway there."
The pairs event concludes Thursday with the free skate.
"I think you'll find there is an easier sense of flow throughout the program," Radford said. "We've just made some little adjustments to free up the program and when we've been skating it in practice, it feels lighter and smoother and it feels easier for us to just let it go and enjoy the moment."
The women's short program is also scheduled for Thursday.
With files from The Associated Press.