Williams turned in an error-strewn performance in her 6-2 6-3 loss to Ekaterina Makarova to suffer her first defeat in Melbourne since 2008 when she lost to Jelena Jankovic in the quarter-finals.
The American, a 13-time grand slam winner, made 37 unforced errors compared to just 17 from the Russian and admitted afterwards she was determined to get her game back on track.
She said: "I am always thinking about things I can do better next time around. That's usually how I respond.
"I'm already thinking about practising as soon as tomorrow [Tuesday]. I can think of 100 things I can do."
Williams has missed the best part of 18 months through injury - she had to sit out last year's Australian Open - and her lack of match practice seemed to catch up with her in the blistering heat.
"I didn't play well and I'm not physically 100%," added the 30-year-old, who was also battling an ankle injury.
"So I can't be so angry at myself, even though I am very unhappy. I know I can play 100 times better than I did this whole tournament."
World number 56 Makarova was delighted to have registered the biggest win of her career.
"I'm surprised because she's a great player and it's really tough to play against her," she said.
"But I was feeling so good and so focused. I played my game and that's it.
"I won against Serena, that's amazing."