Both players fired in centuries in the opening two frames before O’Sullivan won a more tactical third in a session lasting just over half an hour.
The opening frame of the evening session started with a lengthy safety exchange and it was Stevens who was first in, potting a lovely long red off the baulk rail on the way to a magnificent 116 and his first century of the semi final.
Stevens had his chances to get in first in the next but missed three makeable long reds, finally leaving one on for O’Sullivan to capitalise with a break of 130.
Frame 17 proved more of a cagey affair, both players squandering visits to the table. But O’Sullivan held it together at the second time of asking to set up a repeat of the 2008 final.
Second session report
Ronnie O'Sullivan held off a spirited recovery by Matthew Stevens to lead 15-9 in their World Championship semi-final - two frames short of victory at Sheffield's Crucible Theatre.
Stevens managed to haul himself back to 12-9 behind having trailed 11-5 overnight, but could not narrow the gap any further as O'Sullivan ended the session preserving his six-frame lead to leave himself two frames short of a place in the final.
Stevens had chances to win the final two frames of the session against the three-times champion which could have saw him close to 13-11 behind, but missed a simple pink to a middle pocket after benefiting from an awful O'Sullivan safety shot to slot the final red.
O'Sullivan polished off the colours to lead 14-9 before doing likewise in the final frame of the morning session after Stevens missed an easy brown to the opposite middle bag.
Stevens will rue squandering such chances in drawing the session 4-4 when he probably deserved to emerge at least winning the session 5-3.
They play to a finish at 7pm on Saturday night with the first man to 17 frames facing Ali Carter or Stephen Maguire in the final.
O'Sullivan was frozen out in the first frame of the day as Stevens benefited from a fluked red on his way to closing to 11-6 behind.
An early 44 and a closing 39 from Stevens left O'Sullivan on the back foot in the 18th frame with the Welshman winning his second straight frame to make it 11-7.
2003 UK champion Stevens missed a long red at the outset of the 19th frame which was duly punished as runs of 50 and 68 allowed O'Sullivan to steady the ship for a 12-7 advantage.
But Stevens was certainly not slipping quietly into the night - or morning in this case - as a sloppy break-off shot by O'Sullivan enabled his opponent to slot a long red before running in 66.
O'Sullivan tried to recover, but found his situation impossible with the black poorly positioned. Stevens had a skip in his step as he headed for the mid-session interval having reduced his arrears to four frames.
Stevens won a tactical joust and a frame lasting 37 minutes to close to 12-9, but that was as close as he would get with O'Sullivan making a rapid 81 before edging the next two frames despite his opponent putting together a sturdy 51 in the 24th frame.
Second session report
Ronnie O'Sullivan made two centuries in extending his lead over a bamboozled Matthew Stevens to 11-5 in their World Championship semi-final at Sheffield's Crucible Theatre.
Resuming 5-3 behind from last night, Stevens played a wonderful plant to open the reds before making a lovely break of 60 to win the opening frame of the second session.
O'Sullivan seemed likely to move 6-4 ahead with a 61, but Stevens holed a fine long red before making a thrilling clearance of 63 that included a classy pot on a red to the green bag to sneak the frame by one point.
The 11th frame developed a similar theme as O'Sullivan pieced together a lovely 62 only to see a brown stay out when he was only looking for one more colour and a red to leave his opponent needing a snooker.
Stevens forced matters to the final red, but O'Sullivan clipped in the ball he needed to end Steven's three-frame winning run which continued from the final frame of the first session.
O'Sullivan had suddenly regained the momentum that carried him to impressive wins over fellow former world champions Peter Ebdon, Mark Williams and Neil Robertson in reaching the last four.
An immaculate knock of 110 ensured O'Sullivan of a 7-5 lead at the mid-session interval. O'Sullivan (13) is one place above Stevens (14) in the latest world ranking list, but the true gulf in class between the men was all too evident.
Robertson had warned of the dangers of the match becoming a practice stroll for O'Sullivan over such a long distance, and there was another chance for him to hone his skills further in the 13th frame as a knock of 98 handed him an 8-5 advantage.
O'Sullivan left a red hanging over a top pocket at the outset of the 14th frame from a misjudged safety shot. Stevens was snookered and missed the ball attempting to swerve past the blue.
O'Sullivan slotted red before stepping in to make a frame-winning clearance of 90 with an air of effortlessness.
O'Sullivan established a 45-0 lead in the 15th frame with the pink and black out of commission. Stevens had failed to pot a ball for 49 minutes by this stage before he finally picked out a red after O'Sullivan had missed a tricky pink.
He kept alive his hopes until the colours, but seeing a white drop into a middle hole proved fatal as O'Sullivan left him in a telling snooker that paved the way for a 10-5 advantage.
O'Sullivan was seeing the pockets like buckets by now and finished off by winning his sixth straight frame with a run of 113. He is six frames short of reaching the final as his bid for a fourth world title to go with his successes in 2001, 2004 and 2008 continues to gather pace.
They resume at 10am on Saturday morning with Stevens needing an almighty comeback to keep alive his hopes of reaching a third final at the Crucible.
First session report
Ronnie O'Sullivan opened up a 5-3 lead over Matthew Stevens after a turgid first session of their World Championship semi-final in Sheffield.
Welshman Stevens cut a poor imitation of the figure who reached the final of this tournament in 2000 and 2005, but O'Sullivan was also off the boil for most of the evening as both players struggled to adapt to the pace of the table.
Stevens finished the match with a run of 80 - the highest of the night - but there was little else for the capacity Crucible crowd to get excited about on a day when somewhat unusually neither semi-final produced a century break.
Tournament favourite O'Sullivan is chasing a fourth world title, but was not at his best in moving 4-0 clear at the mid-session interval. He did not have to be as Stevens missed a series of easy balls.
Stevens will probably be happier to get out of the evening only two frames adrift before their second session begins at 2.30pm on Friday with the first man to 17 frames reaching the final.
Modest contributions of 52, 66 and 49 were enough for O'Sullivan to trouser the opening four frames with Stevens losing the first frame on the black despite running in 41.
Stevens somehow won back-to-back frames to close to 4-2 behind with the highlight being a 48 in the sixth frame before he was punished for failing to capitalise on another couple of scoring chances to fall 5-2 adrift.
The Carmarthen potter - who is set to be inside the world's top 10 at the end of this tournament - held himself together well in the closing frame of the session, but will probably need to improve his scoring game considerably if he is going to get under O'Sullivan's skin in a best-of-33 frame contest that concludes on Saturday.