It was a quick-fire final with the players rattling through the frames and after the Welshman took the opener, Selby responded with a break of 136 to level the match.
Williams then looked like he was going to grab the third from over 50 points down before a missed regulation red allow Selby back in to get the points he needed.
Williams produced one of the worst break-offs of his career in the fourth but it ended up working out for him as Selby immediately missed a red he would have been expected to get. A break of 51 eventually helped Williams make it 2-2.
A run of 67 then helped Williams go 3-2 as Selby conceded after missing his first shot at an attempt comeback.
Both players had chances in the next before Selby got in and made a 77 clearance to take it to a decider.
But Williams would not be denied the 25,000 euros winners cheque as a break of 65 left Selby requiring snookers before Williams got in again to get the points he needed for the win.
"I came out there with the frame of mind that I was going to play really fast and go for everything," said Williams afterwards.
"It went for shots I wouldn't normally go for and it worked so I might try that again for the next tournament."
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