Chinese youngster Si Jiahui snatched a 5-3 lead in an intriguing first session of his World Championship semi-final showdown with Luca Brecel.
The 20-year-old, who came through qualifying at the Crucible to make the latter stages, lost the first frame, but produced back-to-back clearances of 125, 102 and 97 to punish Brecel for errors with a near-faultless display of break building.
However the Belgian, who beat seven-time champion Ronnie O’Sullivan in the last eight, scrapped his way back to 4-3 down before succumbing in a nail-biting conclusion to the afternoon’s final frame.
In a scrappy start to the opening frame, Brecel enjoyed the benefit of an outrageous fluke when he rattled the pink in the jaws and saw the ball run the length of the table down the cushion before dropping in.
He went on to make 50 before missing the blue off its spot, although Si, the youngest semi-finalist at the tournament since O’Sullivan in 1996, was unable to prevent him from going ahead.
However, the youngster announced himself in style in the second, sinking a tough red to get in before compiling an imperious 125 break, and he repeated the feat to take a 2-1 lead when, after the Belgian had missed the yellow at 33-0, he cleared the table once again in a visit of 102 to complete back-to-back centuries.
Si eased himself 3-1 in front at the mid-session interval after an arm-wrestle in the fourth frame with both players twice having missed difficult reds to the middle pockets before he prevailed 103-29 courtesy of a break of 97.
The world number 80 stuttered briefly, missing a regulation red as he applied right-hand side to the cue ball in an effort to develop the pack after Brecel had erred once again, but eventually tied up an untidy fifth frame 103-3.
Si’s first error of note arrived in the next when, 53 points into another seemingly decisive break, he jawed the black to allow his opponent to the table, and his break of 72 stopped the rot.
Having benefited from a mistake, Brecel handed it back almost immediately, following up a good long red with a careless blue, but the Chinese player was equally profligate, leaving himself unable to see a colour as he attempted to develop the pack and taking seven attempts to hit the nominated brown.
A second snooker and three more misses yielded 12 more points to leave the 28-year-old 38-33 ahead, and he eventually drew back to within one frame of his opponent with a visit of 69.
The final frame of the session unfolded with a sustained safety exchange, but it was Brecel who made the first move with a break of 33 before inexplicably missing the pink, although Si faltered at 43-34.
His opponent rattled a pink to the top pocket which would have levelled the match at 4-4 and saw it roll agonisingly over the middle for the younger man to go two up.
In the evening session, Mark Allen finished the opening stanza of the other semi-final against Mark Selby on a roll, winning the final three frames to lead 5-3 overnight.
A 123 clearance gave Selby the ideal start but uncharacteristic errors in the safety battles in the next two frames allowed his Northern Irish opponent to take control with breaks of 63 and 66.
Selby missed a black off its spot in the fourth frame but still went into the mid-session interval level at 2-2 as it was Allen’s turn to fluff his lines in a tactical exchange.
A break of 68 moved Selby into the lead in an ebb-and-flow session and while Allen levelled, there was a hint of controversy when replays showed he grazed the red he was bridging over to make a tough pot.
The referee did not spot the foul while Allen did not acknowledge it either so - with players so scrupulous in calling to attention any wrongdoing when they are at the table - the suggestion was the left-hander, who has won three ranking events this season, was oblivious to what he had done.
Selby did have a visit to the table thereafter but his rival’s break of 64 left him well adrift and Allen quickly wrapped up the frame. Allen then finished the session with breaks of 60 and 78 to seize the initiative.
