However, the world No.1 was able to regain his composure and draw on his vast experience to claw his way back into the game, before going on to win the tie 10-5.
At 3-0 down, O'Sullivan went on to win six successive frames and convincingly swing momentum back in his favour.
Gilbert then responded well at the start of the second session by taking another two frames early on, but once O'Sullivan refound his rhythm, the 46-year-old didn't look back, winning another four successive frames to secure victory.
30 years after turning professional, Ronnie O'Sullivan heads to Sheffield as world number 1.
Tap ❤️ if you're backing the Rocket to win a seventh world title. pic.twitter.com/AEOZXMYY19
"I am out there playing, enjoying it and just trying to compete. It is like Gladiator. Russell Crowe has a hole in his arm and knows he is going to die, but you just have to find a way. That is what winners and gladiators do.
"To be the best at something it takes graft, time and effort. Sometimes you ask yourself, 'Why am I doing this?'
"I wasn't that bothered, but I was toughened up mentally. I was mentored that way."
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