Canada's Eugenie Bouchard dropped just three games in a stunning win over world number one Serena Williams at the Hopman Cup in Perth.
Bouchard, the world number six, won 6-2 6-1 in 50 minutes to give Canada an early lead over the United States in the mixed team event.
Having required an on-court espresso to get her going in her opening match on Monday, Williams again looked sluggish.
"I don't know what's wrong with me, I'm so tired," said the 33-year-old.
"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.
"I've been trying to rest up. 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."
The American will play Lucie Safarova of the Czech Republic in her next singles match on Thursday.
Bouchard enjoyed her first-ever win against Williams, and joked afterwards: "Maybe she needed another coffee.
"I know she didn't play her best. I'm happy I just stayed with it and held my nerve. It's a good way to start the year - it gives me some confidence."
Vasek Pospisil later beat John Isner 6-3 7-6 (7-4) to give Canada an unassailable 2-0 lead over the US, before Williams and Isner took the mixed doubles 6-3 7-5.
World number two Maria Sharapova began her 2015 campaign with a crushing 6-0 6-1 win over Kazakhstan's Yaroslava Shvedova in Brisbane.
"It certainly felt good to start, after not playing a match for a couple of months," said the 27-year-old Russian.
"Despite all the training, it's such a different feeling to go out on the court and play in front of a crowd."