Siem has been in front from the first day, and stretched his lead another stroke by the end of his third round.
David Horsey and Mikko Ilonen had pegged him back on the front nine, but a run of four birdies in six holes after the turn saw Siem home.
Horsey and Ilonen join Pablo Larrazabal on 11-under-par, leading the chasing pack.
If Siem wins, the 32-year-old is set to jump into the top 50 and with it pick up an invitation for the US Masters.
“Until the 15th I was really happy with the round,” said Siem. “I got over a tough start with tough condition and all the guys just behind me seemed to start really well and almost caught me up.
“I had to hole a few big putts just to stay ahead and then I managed to pull away a little bit. But then 15 and 17 were disappointing because you are really looking to pick up a couple of shots there but I managed to give one away.
“There was pressure there so it was great to make a few putts and pick up four in six holes. It is very nice to have the gap back for the start of the final round tomorrow. I am pretty confident at the moment so I know the swing is there when I trust it and just let it go. That will be very important tomorrow.”