For Ratajski, it is the first time he has won in the tournament after previously failing on three occasions as he knocked out Nathan Aspinall from a set down initially.
To begin the tie, despite only 27% on his finishing doubles, Aspinall edged through the opening set with a 98 average to hold the initial advantage.
Ratajski began the second set with a 98 checkout to hold throw but it was double 18 for Aspinall to leave it all square. The former hit the same target to make it 2-1 with a 96 checkout.
Ratajski though hit back in a deciding leg of the second set to make it 1-1 with a 96 average to set up a decider and he started confident in the second set with a break on double 10.
But Aspinall broke straight back with a 60 checkout at a potentially pivotal time.
On his throw though, he left 157 with Ratajski missing 92 and he made it down to 64 but he couldn't pin it leaving Ratajski another break.
He had the throw as a result to win and Ratajski got off when Aspinall failed to do so until his 10th attempt and 'The Polish Eagle' secured it from there.
While Van der Voort saw off Clemens in a straight sets triumph which didn't tell the full story as he had to level up both sets and take them in deciders. He will now face either best friend Michael van Gerwen or Danny Noppert.
A stunning 104 checkout saw Van der Voort level it up with a break to make it 2-2 and he had the throw for the opening set which he claimed.
Clemens went two bulls for 132 but missed the easy part in double 16 but he went two up against Van der Voort.
But the Dutchman turned it around again with two 16 darters to force a decider which he claimed with a 15 darter in the form of a 61 checkout on double 18 after Clemens couldn't pin 92 to prolong another set.