O'Sullivan is seeking an eighth Masters title, and made three century breaks during the opening four frames, with Fu failing to pot a single ball before the interval.
While the 42-year-old defending champion showed compelling form to clinch victory, he said afterwards that he was under the weather.
O'Sullivan said on BBC2: "To be honest with you, I was really struggling. I don't know if I have got a virus or something and I just feel very dizzy.
"I was just playing off instinct. I was thinking, 'Don't stop, just pot balls'. I was really quite struggling and have been like that for the last five or six days.
"I don't feel ill, I just feel dizzy. I think it is maybe a lack of magnesium or something, that is what my nutritionist told me.
"I have been eating well for the last four months, maybe I have overdone the exercise when I have been at home and made myself a bit ill.
"I know I can get through it, I still feel good physically and you just have to do a job."
In the evening, local favourite Barry Hawkins was beaten 6-4 by Kyren Wilson.
A high-class and closely-fought game looked like it might be tilting in favour of Hawkins before Wilson fluked a red at 4-4, which helped him move within one of the quarter-finals.
Another stroke of luck arrived in the next frame as Hawkins found himself in a snooker and Wilson was able to return to wrap up victory.
Hawkins started the match with a 131 break and added another century in frame three to take an early lead.
Wilson, however, fired in breaks of 109 and 106 and, with two centuries each, it was instead two moments of fortune which ultimately proved the difference.
O'Sullivan is a Sky Bet Price Boost to win the Masters at odds enhanced from 11/10 to 11/8!
Tuesday's results
- Ronnie O'Sullivan 6-0 Marco Fu
- Barry Hawkins 4-6 Kyren Wilson
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