This is the 19th consecutive victory for the unstoppable Italian who has the opportunity in Madrid to make history and become the first player to win five Masters in a row.
Siner led 5:1, Möller managed to return one break, but then the world number one broke his rival's serve for the third time and finished the job in the opening set.
In the second set, only one break made the difference, which Janik achieved in the sixth game.
Interestingly, the talented Dane had two more winners than the Italian (18-16), but also 11 more unforced errors (21-10).
Siner's next opponent will be the winner of the Cameron Nori – Tijago Agustin Tirante duel.
The title in the Spanish capital is being defended by Norwegian Kasper Rudd.
