The former Ryder Cup player birdied the 12th, 14th and 15th to stand three under par, one in front of Scot Stephen Gallacher and Spaniard Pablo Martin.
Graeme McDowell, back on The European Tour three weeks after his US Open Championship victory, parred his first two holes after setting off arm-in-arm with Miguel Angel Jimnez, winner of the Alstom Open de France on Sunday.
McDowell had said on the eve of the event that he will never get sick of being introduced as the US Open Champion - but before hitting his opening drive the official starter opted merely for: "On the tee, from Northern Ireland, Graeme McDowell."
The 30 year old was just off the green in two, but safely two-putted for his four and on the 235 yard next he chipped to two feet after finding the left-hand rough.
Jimnez failed to get up and down from close by, but South African Charl Schwartzel, the third member of the group, chipped in for birdie.
Masters Tournament champion Phil Mickelson, who can replace Tiger Woods as World Number One with a top-two finish on Sunday, was among the later starters.
Fasth did not have the outright lead for long, being joined when Gallacher collected his third birdie on the 371 yard 14th.
It then became a three-way tie when Italian Edoardo Molinari - gunning with his brother Francesco for a Ryder Cup place - eagled the same hole and birdied the next.