Barry Hawkins holds a 5-3 advantage over Mark Williams after the first session of their World Snooker Championship semi-final.
Two-time champion Williams' century in the final frame of the day kept him within two frames of Hawkins in Sheffield.
Hawkins, who finished as runner-up in 2013, started with a brilliant break of 129 and followed it up with 74 before Williams responded with an 88 that included a clever three-ball plant.
The big breaks then briefly went missing, with three successive scrappy frames ending with Hawkins enjoying a 4-2 lead.
Williams then missed a tricky shot - using one rest balanced on top of another - at the start of the seventh, which gave Hawkins the opportunity to make a frame-winning 90.
But Williams responded with a fine 110 to close the gap back to two.
John Higgins and Kyren Wilson - read Thursday's report from their semi-final - will play the morning and evening sessions on Friday, with Hawkins and Williams resuming in the afternoon.