From 9-3 adrift at one stage and in danger of losing the second-round clash with a session to spare, Murphy charged back by taking the next three frames and had O'Sullivan rattled.
But when 2005 Crucible champion Murphy missed an all-important green in the final frame of the session, O'Sullivan pounced to clear and lead 10-6 overnight, a distinctly healthier position than he must have been beginning to fear.
They return on Saturday morning, O'Sullivan needing just three frames to reach the quarter-finals and Murphy knowing only a fast start will give him any chance.
The nearer they come to the finish line, the more worrying the four-frame deficit becomes for Murphy, who at least avoided a repeat of his 13-3 humbling by O'Sullivan at the last-eight stage three years ago.
The 34-year-old began 6-2 adrift on Friday afternoon and made an ideal start with a break of 70 before O'Sullivan responded with 63 and 75 to stretch his lead to five frames.
Even when the next descended into a messy state of affairs, O'Sullivan came out on top and looked to be roaring to a resounding win. The prospect of victory a session early was occupying many minds at the interval, but Murphy showed his focus remained true.
He avoided the possibly ignominy when he ran in a break of 84 and followed up with 73 to trim his deficit to four frames.
Murphy was suddenly on a roll, and an 87 followed, setting up the final frame of the session as possibly pivotal in the context of the match.
Had O'Sullivan's lead been sliced to two frames, the pressure would have been loaded on his shoulders going into the deciding stages, and that was how it looked to be heading when Murphy fluked the yellow.
He had a simple enough green with the rest but missed, and that let O'Sullivan off the hook, in the frame and possibly the match.
In went the five remaining colours, and to the relief of a partisan audience, O'Sullivan could breathe again.
He and Murphy are far from friends and that has been apparent by their lack of interaction during the match, but the opposite was true of the players on the other side of the arena.
Mark Allen took the fight to John Higgins on the table, but Scotland's four-time world champion dug in to be just two frames behind overnight.
Northern Irishman Allen fired 71, 65, 100, 102, 129 and 82 and on that form would have hoped to finish better than 5-3 ahead, the outcome at the end of the session. The pair chatted away in their seats between frames, a stark contrast to the awkward silences between Murphy and O'Sullivan.
Ding Junhui went from 6-2 up on fellow Chinese cueman Liang Wenbo to 9-7 ahead, the gap narrowing in a high-quality encounter that finishes on Saturday afternoon.
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