Luca Brecel 3-5 Si Jiaui
An entertaining opening session in Sheffield was the result of both players displaying some nerves, but it was Si who built a slender lead heading into Friday despite being pegged back.
Brecel, who defeated seven-time champion Ronnie O'Sullivan in the quarter-finals, captured the opening frame but Si quickly wrestled control of proceedings by winning the next four to take a 4-1 lead.
The 20-year-old from China looked to be relishing his place on the big stage but was given a wake-up call when a missed black in frame six allowed Brecel in to reduce the deficit to 4-2.
World number 10 Brecel backed that up with another frame to close the gap to one and then gave up the chance to level the score with a missed pink.
The error duly was pounced upon by Si, who will be relieved to have possession of a two-frame lead heading into Friday's 10am and 7pm sessions.
Mark Allen 5-3 Mark Selby
In Thursday's other semi-final, Mark Allen produced a thrilling display to take a 5-3 lead over world number two Mark Selby.
Allen, the world number three, scored well under pressure to take a 2-1 lead before Selby came back to take the next two frames.
The seventh frame saw Allen take over three minutes to play a red with Selby missing a long shot, before the 37-year-old took another three minutes to play an incredible safety.
To the delight of the Crucible, Allen stormed into a 4-3 lead before taking the eighth and final frame of the session.
