World Grand Prix: Mark Williams beaten by Yuan Sijun in round one
World champion Mark Williams followed Ronnie O'Sullivan out of the World Grand Prix when he suffered a shock defeat to Chinese teenager Yuan Sijun.
Wales' Williams, 43, took the first frame but fell 2-1 behind when 18-year-old Yuan produced a break of 100.
Williams rallied with a 138 of his own to level at 2-2 then edged a frame in front, only for Yuan to compile a 131 and take the decider to win 4-3.
Defending champion O'Sullivan also lost in the first round, to Marco Fu.
Elsewhere in Tuesday's matches at the Cheltenham Racecourse venue, China's Xiao Guodong caused another surprise with a 4-3 victory over Australian Neil Robertson, the world number 10.
Yuan now plays Scotland's Stephen Maguire, while Xiao meets Mark Davis in round two.
Masters champion Judd Trump lost three frames in succession before beating Stuart Carrington 4-3.
Trump will play Tom Ford in round two after he overcame Joe Perry by the same scoreline.