The five-time champion reeled off two century breaks and averaged less than 14 seconds a shot as he raced into an 8-1 lead.
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The only blemish for O’Sullivan came in the seventh frame when Un-Nooh got on the board, but it looks a long way back for the qualifier when the match resumes at 10am on Monday.
Un-Nooh won his only previous meeting with O’Sullivan in 2014 but it was clear from the early stages that he was in for a tough afternoon at the Crucible.
Ronnie O'Sullivan opens with magnificent century break against Thepchaiya Un-Nooh
By the first interval he trailed 4-0 as O’Sullivan reeled off breaks of 101, 85, 115 and 74 in just 40 minutes.
The Rocket followed up with two more quickfire frames after the interval before Un-Nooh got himself on the scoreboard in the seventh frame.
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But a lucky red helped O’Sullivan move further ahead and he closed out a dominant session with a break of 65 to secure a commanding advantage.
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Higgins reaches second round
John Higgins reached the second round with a 10-5 win over Matthew Stevens.
Trailing 6-3 from the opening session, Stevens closed the deficit with a fine 138 break.
But Higgins, who has lost in the final for the last three years, won the next two frames to move closer to victory.
Stevens responded in the 13th frame but couldn't prevent Higgins from securing his place in the next round.
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