Top seed and former world number one Caroline Wozniacki crashed out of the BMW Malaysian Open in the first round Wednesday when she was beaten by qualifier Wang Qiang.
The Dane, who was feeling unwell after suffering a high temperature on Tuesday, hardly chased Wang's returns in the closing stages of the match and lost 2-6, 7-6 (7-1), 6-1.
"I tried my best but I just didn't have energy. I didn't take my opportunities," said world number 10 Wozniacki, who is dating top golfer Rory McIlroy.
"I called for the doctor and the physio to massage my legs. Not much more they could do. I hope to come back again next year."
Wang, who ranks 189th in the world, said she knew Wozniacki wasn't feeling well when she called for a medical time out at the end of the second set.
"I tried to make her run more to take advantage of her illness and it paid off. This is the best win of my career and it will give me more confidence," the 21-year-old said about her first victory over a top-10 player.
No top seed has ever won the four-year-old Malaysian Open.
Wang was the fifth qualifier to reach the second round, joining Thailand's Luksika Kumkhum and Nudnida Luangnam, Zarina Diyas of Kazakhstan and Uzbekistan's Akgul Amanmuradova.
Diyas stunned eighth seed Kristyna Pliskova of the Czech Repubic in three sets to move on to the second round.
Third seed Anastasia Pavlyuchenkova of Russia also came close to crashing out but managed to keep her nerves to edge out compatriot Vera Dushevina 2-6, 6-3, 7-6 (8-6) in the rain-interrupted match.
"It is never easy to play a delayed match. I was ready to finish the match yesterday but the weather was not kind enough," said Pavlyuchenkova about the game, which started on Tuesday before heavy rain forced the players to stop.
"To play under these conditions is a big test and I am happy that I pulled through."
Fourth seed Ayumi Morita of Japan outclassed Turkey's Cagla Buyukakcay 6-1, 6-1.