The 1999–00 season was a very successful one for Williams who won the 1999 UK Championship and the World Championship. These results, along with another ranking title and three runner-up positions, allowed him to capture the world number one position for the first time. In the World Championship final he came from 7–13 behind his fellow countryman Matthew Stevens to eventually win 18–16. He also produced a notable comeback in his semi-final match against John Higgins, coming from 10–14 down to win 17–15. Williams won only one ranking event in the following season, the Grand Prix, with a 9–5 victory over Ronnie O'Sullivan in the final, but he was a runner-up in two other ranking events, the 2000 UK Championship and the China Open. This was enough to retain his number one ranking, although his title defence at the World Championship fell in the second round with a 12–13 defeat to Joe Swail.