Norway's McGrath clocked 51.96 seconds in northern Italy to give himself a chance of claiming a first World Cup win in nearly three years.
The slalom specialist finished 0.62sec ahead of Loic Meillard who could overtake Henrik Kristoffersen at the top of the discipline standings after the second run which starts at 2045 local time (1945 GMT).
Kristoffersen is gunning for a fourth World Cup slalom crown and is 25 points ahead of Swiss Meillard but had a poor first run which left him 1.09sec off the pace.
Frenchman Noel won't have a chance for a third win of the season. he was one of a clutch of contenders flummoxed by a tricky piste and failed to finish his first run.
The 27-year-old only last 10 seconds, continuing his recent troubles after a perfect start to the slalom season.
Noel, who is 30 points behind Kristoffersen in the slalom standings, began the season with two wins from two in the discipline but was then sidelined by an ankle injury at Val d'Isere last month.
