Perth - World No 1 Serena Williams complained of tiredness after being trounced by Canadian Eugenie Bouchard at the mixed teams Hopman Cup on Tuesday.
Williams was humbled by the sixth-ranked Bouchard in straight sets as Canada upset the tournament favourites 2-1.
Williams went down one set against 12th-ranked Flavia Pennetta in her first outing in Perth on Monday but was rejuvenated by a mid-match coffee and went on to beat the Italian.
However, there was to be no coffee-inspired miracle against the sixth-ranked Bouchard, who doubled-faulted five times in her first two services games and yet cruised to an emphatic 6-2, 6-1 win over the 18-time Grand Slam singles champion.
Errors flowed off the Williams racquet during her match with Bouchard and the 33-year-old said she hadn't been feeling at her best since arriving in Perth on Saturday.
"I don't know what's wrong with me, I am so tired, it's so weird," she said when asked if she was suffering from jet lag.
"I can't get my body to move and I feel like I have no energy.
"It is a little frustrating because I know I can play 2000 times better."
But Williams said she wasn't overly concerned by the issue.
"I'm so fit, I can run 10km, I don't know how to describe it," she said.
"I have to figure it out, hopefully I can do better in my next match."
Williams certainly played with renewed vigour in the later mixed doubles rubber.
In beating Williams for the first time, Bouchard rebounded strongly from the disappointment of Canada's opening tie, when she was beaten 6-0, 6-4 by the Czech Republic's in-form Lucie Safarova.
The 20-year-old was delighted with the rapid improvement in her game.
"My goal was to be more active with my feet, try and go for my shots a bit more and be more aggressive," she said.
"I think I did that well, I know she (Williams) definitely didn't play her best but I am happy I stayed with it."
World number 56 Vasek Pospisil secured the tie for the Canadians with an unexpected straight sets win over 18th-ranked John Isner, 6-3, 7-6 (7/4) that was sealed with an ace.
The Americans won the dead mixed doubles rubber 6-3, 7-5.
The result threw Group A at the tournament wide open, with Canada, the USA and the Czech Republic all in contention for Saturday's final.
The Czech pairing Lucie Safarova and Adam Pavlasek are the only unbeaten nation in the group, after a clean sweep against Italy earlier on Tuesday.
If the pair beat the USA on Thursday, they are certain to advance to the final, but if they lose both, Canada and the Americans can qualify for the playoff.
The Czechs put themselves in the box seat for the final when they beat the Italians.
After Safarova notched her second win over a higher-ranked opponent in as many matches in the women's singles against Flavia Pennetta, 7-5, 6-3, the little-known Pavlasek stunned Fabio Fognini in their singles match to secure the tie for the Czechs.
The 239th-ranked Pavlasek fought back from a set down to beat the world number 19, 3-6, 7-5, 6-1.
The Italians forfeited the dead mixed doubles rubber, with Fognini ruling himself out due to a heat-related illness.
"It was the biggest win in my career and it's just amazing that I can play with the top players and I was happy for it," Pavlasek said.
"This is the first time I've ever played someone in the top 20.
"I have to play every point that I can and I still felt good after the first set and I won the match."
Pavlasek said a berth in the final would be a significant incentive for the Czechs when they faced the United States on Thursday.