The 15-time Grand Slam winner, on the prowl for a fourth title at the event, showed no mercy to the Romanian opponent playing with an ankle niggle. William, on a visit to Australia, sat down to a stir in the crowd in time for the closing stages of the rout.
Federer, took only the briefest of notice of his fellow royalty, sticking strictly to business for a clinical victory which marked his his 49th match win at Melbourne Park.
He broke Hanescu breaking five times while never facing a break point in 99 minutes to a standing ovation.
"I knew the danger of Victor," said Federer. "He's a good player. I moved well and I'm thrilled to be through."
Homegrown hero Lleyton Hewitt and Germany's former number two Tommy Haas showed the touch of experience with winning efforts earlier in the day.
The unseeded pair dismissed rivals in contrasting style, with Hewitt administering a master class 7-6 (7-3), 6-4, 6-1 to unschooled American Donald Young and Haas going the distance to put out Serb Janko Tipsarevic, 4-6, 6-4, 6-3, 1-6, 6-3.
Haas, 31, came through on his fifth match point in a marathon victory with 16 aces and 60 winners and a final set lasting for nearly one hour.
Hewitt, 28 and a two-time champion at the majors, holds senior status among tournament survivors, having played in 45 Grand Slams. Haas is just behind on 43.
The 22nd-ranked Hewitt is still battling to return to his best and would surely consider a repeat of his 2005 final at Melbourne Park a decent showing.
He won into a re-run of his late night 2008 Melbourne epic against Marcos Bahdatis, who kissed the court after coming through against David Ferrer of 4-6, 3-6, 7-6 (7-4), 6-3, 6-1.
"I played Marcos last week (Sydney) and I know what to expect. He's in good form and hitting the ball extremely clean. He's gonna be feeling confident after winning (Sydney)."
Novak Djokovic and Nikolay Davydenko staked their claim on possible title runs.
Third-seeded Djokovic, hoping to repeat his 2008 title, dropped a tight first set but then sprinted past Swiss Marco Chiudinelli 3-6, 6-1, 6-1, 6-3.
Federer's good friend was treated for ankle problems and was never able to maintain the upper hand after his opening challenge.
"I was frustrated, especially at start of the match, because he was really giving it to me," Djokovic said. "He was very aggressive and stepping in and using every opportunity that was given to him."
The sixth-seeded Davydenko continued his major run of form, which began last autumn with the Shanghai and London titles and continued into the new year with a win at Doha.
The Russian marched onward over Ilya Marchenko of the Ukraine 6-3, 6-3, 6-0.
"I played better in this second match. I was in control from the baseline. That was important for me," said Davydenko.
The 2008 finalist Jo-Wilfried Tsonga delivered a defeat of father-to-be Taylor Dent 6-4, 6-3, 6-3.
Women's top seed Serena Williams, her right thigh still massively taped, overpowered Czech Petra Kvitova 6-2, 6-1 while her sixth-seeded sister Venus defeated Sybille Bammer 6-2, 7-5.
"I felt like I was moving better and I had better energy," said Williams. "All around I felt a lot better. I could have actually played a lot better today.
"She was definitely taking it to me, though. It was good for me to play a lady like her."
Danish number four Caroline Wozniacki defeated German Julia Goerges 6-3, 6-1 and seventh-seed Victoria Azarenka got away from Swiss Stefanie Voegele 6-4, 6-0.