van Gerwen claimed a commanding win against Mervyn King, Whitlock saw off Colin Lloyd and Thornton completed his first ever success over Raymond van Barneveld.
Dutch star van Gerwen was in imperious form in his 10-3 triumph over King.
The reigning Premier League champion took out four finishes of over 100 and averaged 105 as he set up a last eight tie against Jelle Klaasen.
“I’m really pleased with the win and the performance,” said van Gerwen, the world number two.
“I was glad to improve on yesterday but I had to against Mervyn.
“I hope I can take this form into tomorrow and it could turn out to be a very good day for me.”
Whitlock moved a step closer to a successful defence of his first major title with a 10-6 triumph over Lloyd, which included six 180s for the Australian.
‘The Wizard’, who won the tournament last year, will take on Jamie Caven in Sunday afternoon’s quarter-finals.
“I’m over the moon as I took the couple of chances I had there,” said Whitlock.
“I don’t want to let this title go. I’m like a dog with a bone.
“Colin played very well but I found the shots I needed when I needed them and I’m looking forward to tomorrow.”
Thornton came out on top in an absolute thriller against ‘Barney’, with the Scotsman winning a dramatic last leg decider.
The Scotsman had looked in control when he led 9-7 but van Barneveld forced the last leg shootout, in which both players missed darts to win.
“I thought i had him when i went 9-7 up,” said former UK Open champion Thornton, who faces Ronny Huybrects in the last eight.
“I always do it the hard way but his comeback got into my head a bit so I’m delighted to be through.
“I’m happy going into Sunday after that and I’m looking forward to another great battle.”
James Wade missed out on a quarter-final spot after suffering defeat at the hands of Jamie Caven, while Paul Nicholson was beaten narrowly by Andy Hamilton.
Meanwhile, Huybrechts won through to meet Thornton after knocking out Phil Taylor in a shock victory for the Belgian.
Sunday sees the conclusion of the £200,000 tournament, with the quarter-finals being played out in the afternoon before the semi-finals and final take place in the evening.
By Chris Murphy
Bronze tickets are still available to see van Gerwen, Whitlock, Wade, van Barneveld, Priestley, Huybrechts and Winstanley at OVER THE TOP in Barnsley on September 27 – GET YOURS HERE