(Sports) Serena Williams easily beaten by Eugenie Bouchard in Hopman Cup final

World women's No.1 tennis player Serena Williams was trying to solve a medical riddle on Wednesday after being soundly beaten by Canadian Eugenie Bouchard in the final of Australia's Hopman Cup tournament.

The group A section of the indoor tournament was won by Canada 2-1 after Bouchet disposed of Williams 6-2 6-1 in just 50 minutes in the Western Australian capital of Perth.

Williams, who asked for and received a mid-match espresso coffee to beat Italian Flavia Pennetta on Monday, was flat in her second match of the year.

The 33-year-old battled to hold serve throughout, and appeared lethargic in her attempts to chase down balls, reported.

"I don't know what's wrong with me. I'm so tired," Williams was quoted by the Australian Associated Press after the match.

"It's weird. I can't get my body to move. I feel like I've got no energy.

"It's a little frustrating because I know I can play 2,000 times better."

Williams didn't think the fatigue was purely due to jet lag.

"I've been trying to rest up," Williams said.

"I did a little gym work, but I just felt depleted. I don't know why. I don't know how to describe it.

"I've just got to get my feet moving. I have to figure it out."

Bouchard had lost her previous two matches to Williams, and zeroed in on her opponent's lethargy.

"I know she didn't definitely play her best," Bouchard said at her post match press conference.

"Maybe she needed another coffee. I know she can serve amazing. But I'm happy I stayed with it and held my nerve to the end," she added.

Canada secured the tie win when world No.56 Vasek Pospisil notched an upset 6-3, 7-6 (7-4) win over American John Isner.

"He's one of the best serves in the world right now," Pospisil said.

"You've just got to go with your instincts.

"I'd won our last meeting and I knew what to expect."

The round robin tournament features eight invited mixed gender national teams of two.

Two groups -- A and B -- from eight countries play against each other, with the winners of each group to then play in the final on Saturday night. Endi