Legkov, who was part of the Russian 4x7.5km relay team that finished third at the World Cup opener in Gaellivare last weekend, was a rank outsider at 40/1 for victory in the 10km freestyle in Finland.
However, having won the first-ever Ruka Triple in Kuusamo in 2010, the 29-year-old was dominant once more on Finnish snow as he stormed to a winning time of 21:52.8.
Not even a fightback from Northug could deny Legkov. The Norwegian, who was second in the sprint on Friday, powering through the final kilometres only to miss out on victory by 6.8 seconds.
Northug was over 13 seconds down on Legkov at one stage after a poor start but his efforts were not completely in vain as he leads the Ruka Triple standings with an overall time of 23:49.0.
That is exactly 40 seconds ahead of Alexey Poltoranin, who was 15th in the 10km freestyle, while Legkov is third overall 53.2 adrift of Northug with the 15km classic pursuit left to tackle.
France's Maurice Manificat completed the top three in the 10km freestyle but he is well down in the overall Ruka Triple rankings over a minute behind leader Northug.
However, with temperatures plummeting in to as low as -14 degrees, Legkov admits the conditions are only suiting him ahead of the final event of the competition on Sunday.
"The 10km freestyle was a super competition, I am in good shape and it was very cold but for me it is better when it is cold," said Legkov.
"It was a good competition, my shape got better and better and maybe in the next competition it will be even better."