Runner-up in 2016, Williams withstood the soaring temperatures and a late fightback in Melbourne to beat the 19 year-old Swiss 6-4 6-3.

Up a set and 5-0 in the second, Williams watched Bencic rally to win three consecutive games but despite serving two double faults as she attempted to close out the match, the 22-time grand-slam champion was not to be denied, with Lucie Safarova awaiting in the next round.

There were question marks over Williams heading into the season's opening grand slam after a prolonged spell on the sidelines due to shoulder problems, while she suffered a shock second-round exit in Auckland in the lead up to the Australian Open.

The 35 year-old had her work cut out early as Bencic, who defeated the veteran American at the Canadian Open in 2015, earned two early break points on Rod Laver Arena. Williams, however, dug herself out of a hole by reeling off four consecutive points.

Williams - winner of six Australian Open titles - then fired her seventh winner of the set to earn a break point at deuce in the fourth game and she converted for a 3-1 lead as Bencic sent a forehand long.

But Bencic broke back immediately, screaming out "come on", and she then held serve to level the set at 3-3.

A sensational sliced backhand from Williams stunned Bencic in the ninth game and sparked the former world number one, who ramped things up by breaking again in the 10th to claim the opening set.

All of Bencic's hard work came unstuck early in the second set with a pair of double faults handing Williams the break, a 2-0 lead and the ascendency.

Williams stumbled as Bencic refused to surrender, but the former battled to book her spot in the second round.