The Briton, one of seven European Ryder Cup players competing in Milan this week, was 13-under-par through eight holes when play was suspended for the day because of darkness.
Frenchman Alexander Levy led the way on 14-under through 12 holes, one ahead of Willett, fellow Britons Chris Paisley and Chris Hanson and Italy's Francesco Molinari.
Ryder Cup trio Matt Fitzpatrick, Rafa Cabrera-Bello and Andy Sullivan were among a group of seven players on 11-under.
"It was tricky to read the greens in the fading light," Willett told reporters. "It's always tough to get the pace.
"We need a real good day of weather tomorrow because there are a lot of guys with 27 holes, at least, left to play."
Holders Europe take on United States in the Ryder Cup at Hazeltine, Minnesota from Sept. 30-Oct. 2.
(Writing by Tony Jimenez; Editing by Neil Robinson)
