Now it's straight to the driving range until my fingers are sore, and then I'll come out tomorrow ready to attack," said Siem.
Matti Schmid also carded a 3-under 73 on Thursday and hoped to utilize cleaner morning conditions during his second round.
"I got off to a really good start, but the bogey on the fifth knocked me out of my rhythm.
I found it again on the back nine.
The greens were very soft, and once 100 players have already been through, they become a bit more challenging.
Tomorrow morning, with fresh greens, I'm looking to make a move," said Schmid.
Behind Ortiz on Friday, Adrian Meronk shot a 3-under 69 to reach 8-under overall, while Joaquin Niemann and Abraham Ancer sat at 6-under overall.
Tournament organizers also announced that 25 eagles were converted on Thursday, generating 25,000 euros for the Philipp Lahm Foundation via the Eagles for Education initiative.
"Together with the BMW Group, we are working to empower children for the future and for their lives.
Children have their whole lives ahead of them – and they are our future," said Philipp Lahm, foundation founder.
"Our aim is to provide them with support in three areas: physical activity, healthy eating and personal development."
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Second-round play will conclude on Friday afternoon as the remaining half of the field finishes their rounds in Munich.
