Robertson misses the black frame ball in frame one leading 65-0 with 67 on. Williams gets a chance to steal the frame but misses the black ball on 17 and gives up.
Williams misses on 65 in frame three and Robertson later steals the frame.
Robertson finishes the match with a ton for his second title in Gdynia and third in Poland.
It's his first title since the 2014 Wuxi Classic and he is back up to No.2 in the rankings after Selby.
Robertson's 142 in the last 16 tied for the highest break of the tournament with Maflin and Joyce.
Williams was 3-1 up and only needed the brown ball to go 4-1 up but he missed and Robertson cleared the table. After that he failed to win another frame.