Ronnie O'Sullivan and John Higgins are locked together at 4-4 after an absorbing first session of their World Championship quarter-final.
World number one Higgins took the opening two frames without his fellow three-time champion potting a ball.
'The Rocket' hit back to level at the interval before Higgins won a tactical fifth and a superb 121 made it 4-2.
But O'Sullivan responded with breaks of 89 and 105 to gain parity. The pair resume battle at 1430 BST on Wednesday.
Their fifth Crucible meeting - they both have two wins apiece - started in cagey fashion before breaks of 41 and 53 gave Higgins the opening frame.
A run of 66 was enough for the Scot to make it 2-0 but O'Sullivan, who barely had a chance early on, found some fluency in the next two frames.
He looked favourite to make it three in a row on the resumption when 61-33 ahead but missed a difficult red to the bottom corner and Higgins cleared to the black to take the frame by a point.
A magnificent 121 in the sixth suggested the 1998, 2007 and 2009 champion was about to take full command but O'Sullivan's 105th Crucible century, and the 50th of this year's championship, saw the two rivals remain level ahead of two further sessions on Wednesday afternoon and evening.
In the other quarter-final to get under way on Tuesday, Chinese star Ding Junhui upset pre-match expectations by taking a 5-3 first-session lead over Mark Selby.
Selby, up to number two in the provisional world rankings and in superlative form in Sheffield to date, opened with his eighth century of the tournament, a break of 124.
But Ding, capitalising on a series of errors from the Leicester cueman, took the next four frames, helped by breaks of 58, 63 and 65.
Selby found his fluency with an 87 to make it 4-2 only for Ding - playing in his first Crucible quarter-final - to respond with 77 to restore his three-frame advantage.
But a 74 break saw 2007 finalist Selby stay in touch, two frames adrift.
They resume on Wednesday at 1000 BST with the best-of-25 contest to conclude on Wednesday evening from 1900.