As world champion, Williams won the second event of the new season, the 2018 World Open. After recovering from 0–3 behind in his quarter-final against Jack Lisowski to win 5–3, he than came back from 2–5 down in his semi-final against Noppon Saengkham to win 6–5. In the final against David Gilbert, Williams again recovered from a sizeable deficit of 5–9 to win the match 10–9. At the 2019 World Championship, Williams' defence of his world title ended in the second round, with a 13–9 loss to Gilbert.