Following Monday’s opening session, Welshman Williams, the 49-year-old fifth seed, was ahead 5-4 in his first-round encounter against Si.
But the semifinalist from the previous year won four straight to lead 8-5 before Williams lost two frames.
Si was the next to move ahead 9–7 and had a fantastic opportunity to win the match before missing a pink when leading 39–14, which gave Williams hope.
The winner from 2000, 2003, and 2018 led 9–9, but Si made the game-winning break of 77 after he left a red on.
It has been an unexpected tournament, as Williams exits early along with first seed Luca Brecel, fifth seed Mark Selby, ninth seed Ali Carter, tenth seed Gary Wilson, and eleventh seed Zhang Anda.
After defeating Zhang of China on Sunday, world number 44 Jak Jones will play another Welshman in the round of 16.
