World champion Mark Williams joined Ronnie O’Sullivan on the list of first-round exits at the World Cup Grand Prix after a 4-3 defeat to teenager Yuan Sijun.

Chinese star Yuan, 18, had already shown a glimpse of his potential this season with a fine run to the quarter-finals of the China Championship, which included a win over compatriot Ding Junhui, while last week in Germany he took the scalp of John Higgins at the Berlin Masters.

In Cheltenham, Yuan held his nerve to win a final-frame decider after a high quality and pulsating battle with the Welshman.

They exchanged the first two frames before Yuan went ahead 2-1 courtesy of a 100 break only for veteran Williams to come straight back with a fine 138 clearance of his own to square things up.

The left-hander looked to have landed a piercing blow as he grabbed frame five on a re-spotted black to move within one of victory but once more it was Yuan that added a 131 knock to force the decider.

Williams twice missed half-chances into middle pockets in the seventh frame, allowing his teenage opponent in for crucial scoring opportunities as he built a healthy advantage in frame seven and, with two bites at the cherry, did enough to secure his latest big-name knockout.

Yuan said: “My form has been pretty good recently and I knew I had the chance to beat top players. I was a little bit surprised to qualify for this event after a tough first season on tour. The World Grand Prix wasn’t even in my thoughts six months ago.”

Next up for Yuan is a clash with Stephen Maguire on Wednesday evening and is 2.30 to win there, with the Scot favourite at 1.80.