Viljanen won three SDC Tour events in 2015 and finished top of the Order of Merit, thus qualifying for the 2016 PDC World Championship. After winning 2–1 against Sven Groen in the preliminary round, he was beaten 3–0 by Kevin Painter in the first round. Viljanen made his debut at the 2016 World Cup of Darts, representing Finland with Marko Kantele and they lost 5–1 to Wales in the opening round. Another three SDC titles saw him qualify for the 2017 World Championship and he saw off Ross Snook 2–0 in the preliminary round, but could only win one leg against world number one Michael van Gerwen during a 3–0 first round loss. Viljanen missed two match darts in the first round of the World Cup as he and Kantele were knocked out 5–4 by Wales.