The defeat ended Littler's bid to win the only major title missing from his collection, with Ratajski advancing to the semi-finals, where he will face Martin Schindler.
The other semi-final will be contested by Gerwyn Price and Ross Smith, with action set to resume at the Westfalenhalle by 19:10 CET.
Results
Quarter-finals
Ross Smith 10-6 Luke Woodhouse
Gerwyn Price 10-3 Stephen Bunting
Martin Schindler 10-1 Jonny Clayton
Krzysztof Ratajski 10-6 Luke Littler
Semi-finals
Martin Schindler vs Krzysztof Ratajski