O'Sullivan complained about the worst kicks of his career after an error-strewn start to the session, which the Rocket eventually had to settle for edging 5-3 despite dominating his out-of-sorts opponent.
After both players suffered a series of bad cue ball contacts in the third frame, O'Sullivan told referee Paul Collier: "I've never had kicks like that before in my life."
Tournament officials took the unusual step of changing the balls during the mid-session interval and the kicks appeared to subside, with O'Sullivan fashioning an overnight advantage which ought to have been much more comprehensive.
Players and pundits including six-time runner-up Jimmy White have suggested the warm weather may be a factor in the increased incidences of contact issues this year.
White said on Eurosport that "it might be the humidity outside", while O'Sullivan himself referenced the "damp conditions" after his second round win over Ding Junhui.
A World Snooker Tour spokesman confirmed that "humidity is bound to be a factor", but pointed out that kicks have been "dramatically reduced" in recent years following the introduction of an anti-static cloth.
O'Sullivan had been the first to suffer in the opening frame when a bad contact on a black brought his break of 59 to an end, although a brilliant long red belatedly helped him nudge ahead.
Both players made uncharacteristic errors in the following frame, with Selby's proving the most costly when he caught a red on the lip of the pocket on a break of 39, and some fine long-potting from O'Sullivan helped him extend his lead.
Selby held on to win a near-farcical third frame in which the three-time champion was seen to express his frustration, and after which O'Sullivan engaged Collier in conversation as his opponent briefly left the arena.
O'Sullivan went 3-1 ahead with a break of 85 before the interval, and while the kicks proved negligible upon the resumption, the error-strewn action continued as Selby missed the simplest of greens to help sweep O'Sullivan into a 4-1 lead.
But Selby responded in combative fashion by winning a sixth frame which hinged on his potting of a red to the middle which O'Sullivan had earlier refused.
O'Sullivan fired a break of 58 to go 5-2 ahead but Selby clawed his way back from a 51-point deficit in the final frame of the evening to finish a largely forgettable session 5-3 behind.
Earlier, Anthony McGill moved closer to a first World Championship final by opening up a 6-2 lead in the opening session of his semi-final clash with eighth seed Kyren Wilson at the Crucible.
Wilson was almost unrecognisable from the player who polished off defending champion Judd Trump on Tuesday as a series of missed pots and safety errors gifted qualifier McGill the chance to continue his dream run in the tournament.
After an epic first-round triumph over 13th seed Jack Lisowski and subsequent successes against Jamie Clarke and Kurt Maflin, McGill now stands a strong chance of joining Stephen Hendry, John Higgins and Graeme Dott as the only Scottish finalists in the Crucible era.
Wilson started the semi-final a heavy favourite after his stunning performance against Trump, but he was slow to get out of the blocks and McGill instantly seized advantage with breaks of 83 and 78 respectively.
Even when Wilson got his chance he all too frequently missed simple pots or ran out of position, a mistake on the green in the third frame abruptly ending a break of 55 and allowing McGill to step in and extend his lead.
Wilson belatedly got a frame on the board and although McGill took the next two to lead 5-1, a simple missed blue in the seventh proved the difference between the Scot going further ahead and Wilson coolly clearing the colours to reduce his arrears to 5-2.
However, any hope of it being a turning point for Wilson were dashed as the nerveless McGill polished off a highly satisfactory session from his perspective with a break of 92, securing his advantage ahead of their resumption on Thursday morning.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies