Siem defeated Belgium’s Thomas Detry 3&2 in the last 16 at Bad Griesbach Golf Resort before recovering from two down with four to play to beat England’s Robert Rock on the 19th.
Asked about the prospect of matching major champions Langer and Kaymer in winning a European Tour event on home soil, Siem admitted: “It’s shivers already.There’s a lot of golf to play still. It would be fantastic to win on home soil. It would be very special for me.”
Siem will face Sweden’s Johan Carlsson in the last four after the 32nd seed beat Marcus Fraser 4&2 in the last 16 and then ended Anthony Wall’s hopes of a successful title defence in the quarter-finals.
Wall, who had earlier beaten Ryder Cup hero Jamie Donaldson to record his ninth straight victory in the event, birdied the 17th to get back to all square against Carlsson, but then found the water with his third shot to the 18th and was forced to concede.
The other semi-final will see Spain’s Adrian Otaegui take on compatriot Alejandro Canizares after the pair enjoyed hard-fought quarter-final wins over Alexander Knappe and Chris Paisley respectively.
Meanwhile, defending champion Falko Hanisch will face 2016 Junior Open winner Pedro Lencart in today’s final of the 91st Boys Amateur Championship at Nairn.
Hanisch set up the chance to become only the third player to win back-to-back championships – the first since 1930 – thanks to a 4&3 victory over England boys internationalist Robin Williams. He overcame Benjamin Jones by one hole in his earlier quarter final tie.
Hanisch, who lifted the trophy at Muirfield after an enthralling final with Spain’s Alejandro Aguilera that went to the 37th hole, was never behind in the match against Williams.
Lily May Humphreys won the Girls British Open Amateur Championship at Enville, Staffordshire, defeating Emilie Overas 7&5 in the 18-hole final to seal the biggest win of her career.
The 15-year-old has earned an exemption into Final Qualifying for the Ricoh Women’s British Open next year at Royal Lytham & St Annes.