Whitlock didn’t miss a single attempt at a double in a thrilling match, which saw him win the last four legs to fend off an impressive Thornton, who averaged over 103.
The world number three took maximum points to give himself a fighting chance of avoiding relegation, having suffered defeat in each of his first four games.
“It was such a relief to win and I’m really happy to be off the mark,” said Whitlock, who will compete in the UK Open finals on Friday.
“I was under the weather at the start of the season and I’ve struggled to shake it off but there are no excuses. I’ve been under par and I was pleased to put that right tonight.
“Robert played extremely well and I had to hit all my doubles to win. It is a great achievement to have a one-hundred-percent finishing statistic but I wouldn’t have won without it. That’s how hard this Premier League is.
“The important thing was get the win and now I can look to build on it and take that performance into the weekend.”
Thornton kicked off with a 180 and took out 86 to break Whitlock’s throw immediately before ‘The Wizard’ replied in kind with a neat 90 finish.
Whitlock then started leg three with his first 180 and he took out 70 to win it but Thornton’s second maximum helped him draw level, despite Whitlock also scoring his second 180.
The Australian fired in a 171 and produced a sensational 150 finish to move back in front by the odd leg in five.
The pair both started the sixth leg with their third 180s, with Thornton going on to restore parity by pinning double top to complete an eleven-darter.
He followed that by again going off with a maximum as he grabbed a break of throw, before bedding tops to move two legs clear.
Whitlock took out 61 to pull a leg back and then pegged double top for a crucial break of throw, squaring up the match at five legs apiece.
And when Thornton missed two darts at double four, Whitlock stepped forward to clean up 60 and claim his first point of the campaign.
‘The Wizard’ made sure he would take two points by taking out 81 to seal victory with his opponent waiting on 24.
By Chris Murphy