(CNN) -- She suffers from an autoimmune disease, but seven-time grand slam winner Venus Williams refused to blame her health for an early exit at the Australian Open.
The former world No. 1 came into the year's first grand slam in good form after reaching the final of a warm-up event in Auckland in New Zealand, but despite winning the first set against Russia's Ekaterina Makarov, Williams slipped to a 2-6 6-4 6-4 defeat.
Williams, ranked 37th in the world, was diagnosed with Sjogren's syndrome -- which causes joint pain and can deplete energy levels -- in 2011. She hasn't won a grand slam since Wimbledon in 2008.
But the American, who will compete in the doubles event alongside her sister and top seed Serena Williams, refused to look for excuses after her defeat by the 22nd seed Makarova, who will face U.S. world No. 141 Irina Falconi in the second round.
"I think that's a factor for any professional athlete, so I don't think I'm any different from anyone else," Williams, who made 56 errors during the match, told reporters when asked if her health was the reason for her defeat.
"The last 12 months I have had issues, but this year I definitely am looking forward to having a good run and feeling well."
Williams will now focus her energies on supporting Serena, who wasted no time in wrapping up a 6-2 6-1 win over Australian 17-year-old Ashleigh Barty.
The biggest upset of the opening day saw sixth seed Petra Kvitova slip to a 6-2 1-6 6-4 reverse against Thailand's world No. 87 Luksika Kumkhum.
It was Kumkhum's first ever match with a top 10 player and her victory means she will at least equal her best grand slam performance to date -- reaching the second round in Melbourne last year.
"I didn't play well," conceded Kvitova, a Wimbledon winner in 2011. "I didn't play my game that I really tried all off season to work on. It was a great off season. I was really excited to be here, feeling good.
"But I think that probably I wanted too much, and then everything just fell down."
Australian Sam Stosur delighted the crowd by chalking up a 6-3 6-4 win over Klara Zakopalova of the Czech Republic.
Seventeenth seed Stosur won the U.S. Open in 2011 but has never been beyond the fourth round of her home grand slam.
The weather looks set to play a major part in the tournament, with temperatures expected to top 100 degrees Fahrenheit on Tuesday after relatively mild conditions on Monday.
Organizers are reportedly preparing ice vests and ice towels to keep players cool, but Stosur felt perfectly at home in the heat.
"It was pretty hot out on court," said the 29-year-old Australian. "You get that response from the court and balls when it's hot and the sun is shining.
"I was jumping and using my spin and all that whenever I was able to, so that was nice."
Fourth seed Li Na, beaten by Victoria Azarenka in last year's final, admitted she knew nothing of her Croatian opponent Ana Konjuh before registering a comfortable 6-2 6-0 success.
Li's reward for beating the world No. 239 is a second round meeting with Switzerland's Belinda Bencic.
Fresh from picking up her first title in two years in Auckland, 14th seed Ana Ivanovic beat Dutchwoman Kiki Bertens 6-4 6-4.
There was also joy for Germany as both world No. 15 Sabine Lisicki and Angelique Kerber, seeded ninth, advanced courtesy of wins against Mirjana Lucic-Baroni and Jarmila Gajdosova respectively.