|Betfred World Championship|
|Venue: Crucible Theatre, Sheffield Dates: 31 July-16 August|
|Coverage: Watch live on BBC One, BBC Two, BBC Four and Red Button, with uninterrupted coverage on BBC iPlayer, BBC Sport website and BBC Sport app. Full details and times.|
Ronnie O'Sullivan earned a 5-3 lead against a battling Mark Selby in the World Championship semi-finals.
O'Sullivan, 44, looked comfortable as he capitalised on 37-year-old Selby's errors to take a 4-1 lead in the first session of the best-of-33 match.
But Selby ended Wednesday just two frames down after a strong finish.
Earlier, Scotland's Anthony McGill took a shock 6-2 lead over Kyren Wilson in his first World Championship semi-final appearance.
World number 39 McGill, 29, started strongly to take a 3-0 lead and ended the session in similar fashion with a break of 92.
O'Sullivan and Selby's semi-final continues from 14:30 BST on Thursday, with Wilson and McGill starting at 10:00 BST.
Selby rallies to reduce O'Sullivan's lead
Five-time champion O'Sullivan, who called his quarter-final performance against Mark Williams "embarrassing", impressed with his long game as he won the first two frames.
Both players struggled with cue-ball contacts, possibly because of increased humidity, and Selby missed an easy blue in the third but the three-time champion claimed his first frame nevertheless.
O'Sullivan hit the highest break of the match, 85, to go in to the mid-session break with a 3-1 lead.
New balls were brought out after the break to try and counteract the contact issue but that did not help Selby as he missed a simple green and O'Sullivan took the fifth frame.
Selby, playing in his first World Championship semi-final since he won the title in 2017, edged the sixth to make the score a less daunting 4-2 before O'Sullivan extended his lead once more.
Both players struggled to close out the final frame but it was Selby who eventually came back from a 51-point deficit to end the day 5-3 down.
'Selby will feel like he's almost won' - Analysis
Seven-time world champion Stephen Hendry on BBC TV
Mark Selby will feel like he's almost won that session. Ronnie O'Sullivan might be a bit disappointed.
These players are so experienced. They know there's a long way to go.
I think Ronnie would have taken a 5-3 lead before the start of tonight.
It was a disappointing session in terms of the standard. I would have expected more but there were problems with the balls in the first four frames.
They are two of the best players in the world. There is pressure out there. These guys know that the winner of the World Championship will probably come from this match.
McGill unfazed on semi-final debut
McGill looked comfortable stepping up to the one-table set-up at the Crucible as he took the first three frames, which included breaks of 83 and 78.
Wilson - who beat defending champion Judd Trump in the quarter-finals and is ranked 31 places higher than McGill - eventually got on the board in the best-of-33 match but the Scot took control again after the mid-session break to go 4-1 up.
The Englishman's error count increased as the pressure mounted and a break of 69 helped McGill take the sixth frame.
After a break of 45, Wilson gave McGill a chance to win the seventh but he missed a straightforward blue to the middle and the opportunity to take a 6-1 lead.
McGill then came close to his first century break of this year's tournament and had to settle for a 92 to end the session.
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