O’Sullivan made two century breaks as well as scores of 98, 90, 62 and 61 as he won six frames in a row.
The three-time champion has now made eight century breaks at the Crucible this year — the most of any player.
Stevens, who started the second session 5-3 behind, won the first two frames but hardly got a look in after that. It was a scintillating session of snooker from O’Sullivan, who was ruthless when given a chance.
The high-scoring breaks were signs that “The Rocket” is determined to capture his fourth world crown. Much like his match against Mark Williams, O’Sullivan caused the damage in the second session and the “Welsh Dragon” would require a near perfect third session on Saturday morning to stay in touch.
Stevens, who has not beaten O’Sullivan since 2003, found a plant in the opener and was able to spread the reds on his way to compiling a frame-winning 60 break.
Having made a 62, O’Sullivan looked favourite to take the next frame but a missed red into the yellow pocket let Stevens in, whose brilliant clearance allowed him to nick the frame by a point and level the match at 5-5.
The Englishman (36) soon put that out of his mind by taking the following two, which included a superb century.
O’Sullivan reasserted his control on the match by making another controlled break, this time a 98 and in turn extended his lead to 8-5.