A ferocious James Blake thrashed world number one Lleyton Hewitt in sensational style on Saturday to win the Hopman Cup for the United States.

The American, 27 places below Hewitt in the world rankings, obliterated the Australian's defences in just 64 minutes with searing serves and forehands to triumph 6-3, 6-4 at the Burswood Dome.

Women's world number one Serena Williams had earlier set the U.S. up for victory, blasting Alicia Molik 6-2, 6-3 but not even she could have expected Blake to punish Hewitt so ruthlessly.

Against all odds the stylish American produced winner after winner to grind an under-par Hewitt into the ground, his first victory against the Australian in five meetings.

"I was here a few years ago and didn't really feel I belonged...I was kind of a substitute I think," Blake smiled. "Well now I am back, and I really feel I belong."

"It's the first time I've taken out a reigning number one. Lleyton maybe wasn't on his best form but I'll take it -- my kids and grandkids won't know that when I tell them I beat the number one in the world."

The 23-year-old barely missed a shot at the Burswood Dome, firing groundstrokes into the corners, onto the lines and running Hewitt ragged in the indoor arena.

Smiling throughout, Blake's demeanour was in stark contrast to that of the sweating, scowling Hewitt who could find no answer to Blake's firepower.

CONFIDENCE KNOCK

The defeat, two days after losing to Jiri Novak in a group match and coming just a week before the Australian Open in Melbourne, marks the first time Hewitt has lost two matches in succession since last March and April when he lost in Miami and Monte Carlo.

The United States had won the mixed team event just once from five finals before Williams and Blake teamed up this year and the pair certainly plan on returning next year.

"I have had so much fun here," Williams said. "I just must come again."

"I'm glad to hear that, Serena," Blake added. "I am going to book my plane right now."

Hewitt and Molik had also been looking to clinch Australia's second triumph.

But Williams dealt the hosts an early blow and brought Molik back to earth with a devastating display.

The 21-year-old Molik had enjoyed the best results of her career coming into the final, but was helpless as Williams put the U.S. into a 1-0 lead.

A pair of breaks in both sets were enough as Williams cantered past the home hope without barely breaking sweat.

"It's really great -- it's a wonderful team atmosphere and I have really enjoyed it, really enjoyed partnering James," said Williams after the winners collected cheques for A$120,000 ($67,610) each and a pair of diamond-encrusted golden tennis balls.