World No.1 Serena Williams was left searching for answers after her mystery tiredness continued during her shock 6-2 6-1 Hopman Cup loss to Canadian Eugenie Bouchard on Tuesday night.
Williams, who needed a mid-match coffee hit to get over Italian Flavia Pennetta on Monday, again looked flat in her second match of the year.
The 33-year-old battled to hold serve throughout the 50-minute contest, and appeared lethargic in her attempts to chase down balls.
"I don't know what's wrong with me. I'm so tired," Williams said 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 two thousand times better."
Williams arrived in Perth from America on Saturday, but she doesn't think the fatigue is purely due to jetlag.
"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. Hopefully I can do better in the next match."
Bouchard, who had lost her previous two encounters to Williams, capitalised on her opponent's lethargy with some sharp ground strokes.
"I know she didn't definitely play her best," Bouchard said.
"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."
Canada secured the tie win when world No.56 Vasek Pospisil notched an upset 6-3 7-6 (7-4) win over big-serving 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."
Williams perked up for the mixed doubles, combining with Isner to secure a 6-3 7-5 win.
The US must beat the Czech Republic on Thursday night if they are to secure a spot in Saturday's final.
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