Wozniacki saw off third seed Sabine Lisicki of Germany while a retirement from Swiss semi-finalist Stefanie Vogele put Beck, who was a set up, through.
The Kremlin Cup women’s singles title meanwhile will be decided by fifth seed Simona Halep, who beat Anastasia Pavlyuchenkova 6-2 6-1, and seventh seed Samatha Stosur who also earned a straight-sets last-four win over Russian opposition by despatching Svetlana Kuznetsova.
WTA Luxembourg semi-finals
1-Caroline Wozniacki (Denmark) beat 3-Sabine Lisicki (Germany) 6-4 6-4
Annika Beck (Germany) beat Stefanie Voegele (Switzerland) 7-5 1-0 (Voegele retired)
Kremlin Cup women’s semi-finals
5-Simona Halep (Romania) beat Anastasia Pavlyuchenkova (Russia) 6-2 6-1
7-Samantha Stosur (Australia) beat 8-Svetlana Kuznetsova (Russia) 6-2 6-4