PERTH, Australia -- Serena Williams stepped up her Australian Open preparations with a straight-sets victory over Britain's Katie Boulter on Thursday to complete an unbeaten Hopman Cup campaign.

Williams cooled off after a strong start but prevailed with a 6-1, 7-6 (2) victory in 78 minutes to level the tie between the United States and Britain in Group B.

The 37-year-old's hopes for a record third Hopman Cup title ended when the United States were eliminated after losses to Greece and Roger Federer-led Switzerland.

Williams slowly built momentum in her first tournament back since melting down at the U.S. Open Final in September as she eyes a record-equaling 24th Grand Slam singles title in Melbourne.

"It was definitely good to play three singles matches here, and the mixed (doubles)," Williams said.

A sluggish Williams started slowly in victories over Maria Sakkari and Belinda Bencic, but breezed through the first set against a beleaguered Boulter in just 24 minutes.

Watched on by her daughter Alexis Olympia, the sweet-hitting Williams found her groove through an array of powerful forehand winners to overwhelm the 97th-ranked British player.

The one-sided contest took a surprising turn when Boulter broke Williams early in the second set but she was unable to consolidate on serve. Williams was made to work before putting the foot down in the tiebreak.

Earlier, Cameron Norrie upset Frances Tiafoe 7-6 (4), 6-0. It ended a miserable tournament for the 20-year-old Tiafoe, who won just one set in his three matches.

The No. 91-ranked Norrie was impressive in Perth, highlighted by defeating Greece's No. 15-ranked Stefanos Tsitsipas in his opening match. (AP)