Seven-time champion Ronnie O'Sullivan is level at 4-4 with Zhao Xintong after an entertaining opening session to their World Championship semi-final. China's Zhao, 28, a former UK champion who is competing as an amateur, showed few signs of nerves having reached the single table set-up for the first time, racing into a 2-0 lead with breaks of 60 and 62. However, O'Sullivan, 49, appearing at this stage for the 14th time, clicked into gear and responded with three consecutive half centuries of his own as he took the next three frames to lead 3-2. The players then traded frames, with Zhao breaking the right corner pocket potting the black while crafting an 86 to draw level at 4-4. Their best-of-33 frame match, due to be played over four sessions, will resume on Friday at 10:00 BST. Three-time Mark Williams and Judd Trump, the 2019 Crucible champion, start their semi-final tonight at 19:00 BST.