Whitlock won the £200,000 tournament last year, defeating Wez Newton in the final to clinch his maiden major title in the PDC.
The Australian ace faces Belgian star Kim Huybrechts on the opening night of the four-day event in Mulheim, Germany.
‘The Wizard’, who defeated Huybrechts in the semi-finals on his way to glory last year, has set his sites on repeating the feat but knows that he has a tough task on his hands.
“It’s a good feeling to be going back to a major tournament as the defending champion and hopefully it can inspire me to another great tournament,” said Whitlock, the world number five.
“Winning the European Championships last year was a very special and proud moment for me.
“Kim is a really dangerous player and I know I will have to be at my absolute best to get the right result. If I can come through with a victory against Kim, it’s the type of game which will set me up nicely for the rest of the tournament.
“The talent in darts at the moment is unbelievable but I will be doing everything I can to hold on to my title.”
World number four Wade will now face Webster after his a original opponent Gary Anderson had to withdraw through illness.
Welsh left-hander Webster is the automatic replacement as the highest ranked player who had not already qualified.
“Obviously I would like to have been there in normal circumstances,” admitted Webster.
“But this is a bonus for me and I will go to Germany with absolutely nothing to lose. I know the enormity of the task when I face one of the best players in the world on Thursday but I’m not putting any pressure on myself so we’ll see how it goes.”
2012 UK Open champion Thornton continues his hunt for a second PDC major title when he takes on Brendan Dolan, while van der Voort faces four time champion Phil Taylor.
Raymond van Barneveld is also in action on the opening night. The Dutch legend takes on Tomas Seyler.
The second round will be played across two sessions on Saturday, with the quarter-finals, semi-finals and final taking place on Sunday.
By Chris Murphy