Venus Williams upset top-seeded Victoria Azarenka 6-2, 6-4 on Tuesday to advance to the third round of the Pan Pacific Open.
Williams, making her first appearance in the Tokyo tournament since 2009, took advantage of six-double faults by Azarenka, who struggled with her game from the outset at Ariake Colosseum.
Azarenka said after Tuesday's match that she wasn't well and hadn't been able to practice for three days because of her condition.
"She didn't seem like herself at all," Williams said.
Hitting the double-faults, I could tell she wasn't feeling her best. I am not sure what was bothering her but I hope she feels better
"Hitting the double-faults, I could tell she wasn't feeling her best. I am not sure what was bothering her but I hope she feels better."
Azarenka, who lost to Serena Williams in the final of the U.S. Open, broke Williams to make it 4-4 in the second set after trailing 4-2.
But Williams came right back in the next game to go up 5-4 ad then served out to win the match in 1 hour, 21 minutes.
"I didn't sleep at all last night," Azarenka said.
"You can't play at 20 percent against one of the top players. I've been feeling bad for a couple of days and it just got worse today."
In 2011, Williams withdrew before her second-round match at the U.S. Open because of Sjogren's syndrome, an autoimmune disease and came into the Tokyo tournament 63rd in the world rankings.
"I've had some issues to say the least," Williams said.
"But I like to think that builds character and I've got a whole lot of character. I'm getting better every tournament, my serve is better, my back is better, a lot of things are getting better."
Williams next faces No. 13-seeded Simona Halep of Romania, who defeated Germany's Andrea Petkovic 7-6 (4), 6-3.
In other second-round matches, second-seeded Agnieszka Radwanska of Poland defeated Canada's Aleksandra Wozniak 7-6 (5), 6-0. Fourth-seeded Caroline Wozniacki of Denmark overpowered Flavia Pennetta 6-2, 6-3.
Svetlana Kuznetsova also advanced by beating third-seeded Sara Errani of Italy 6-4, 6-4.
Kuznetsova, a two-time major winner, hit seven aces in the match. The Russian and will next face 15th-seeded Sorana Cirstea of Romania. Cirstea beat Japanese wild-card entry Misaki Doi 6-1, 6-2.
Canada's Eugenie Bouchard defeated ninth-seeded Sloane Stephens of the United States 5-7, 7-6 (7), 6-3. Bouchard will next play sixth-seeded Jelena Jankovic of Serbia, who beat Japan's Ayumi Morita 6-4, 6-1.
Other winners include Samantha Stosur, Ana Ivanovic, Dominika Cibulkova and Madison Keys.