Top seed Jelena Jankovic retired from the Bangkok Open after suffering from heat illness in her first round match with China's Zi Yan on Wednesday.
On a humid evening in the Thai capital, Jankovic let slip a one set lead to the gutsy Chinese and called it a day after a medical timeout, having lost the second set tiebreak.
The world number three had looked in good form during the first set, racing through her service games and unloading a torrent of topspin-loaded drives from her forehand and backhand to win 6-4.
Jankovic was dictating the match from the baseline, forcing frequent errors from Zi, whom she had darting across the court for most of the first set.
But the determined Chinese battled back in the second, breaking Jankovic after four games as the heat and a series of marathon rallies took their toll on the Serb, who told medical staff she was experiencing breathing difficulties.
Her retirement was the latest setback for Thai tennis following high-profile withdrawals of Rafael Nadal, Andy Roddick and Novak Djokovic at the ATP Thailand Open two weeks ago.
In earlier matches in Bangkok, defending champion Vania King beat fellow American Jill Craybas 6-1, 3-6, 6-2 to progress to the second round.
Frenchwoman Camille Pin defeated unseeded Czech Klara Zakopalova 6-3, 7-6 to go through, along with Taiwan's Chan Yung-jan, a 6-3, 6-3 winner over Slovakian Magdelena Rybarikova.
Results:
Second Round:
7-Flavia Pennetta (Italy) beat Casey Dellacqua (Australia) 2-6, 6-3, 7-5
Camille Pin (France) beat Klara Zakopalova (Czech Republic) 6-3, 7-6(1)
First round:
Yan Zi (China) beat 1-Jelena Jankovic (Serbia) 4-6, 7-6(4) (Jankovic retired)
Vania King (U.S.) beat Jill Craybas (U.S.) 6-1, 3-6, 6-2
Chan Yung-jan (Taiwan) beat Magdalena Rybarikova (Slovakia) 6-3, 6-3
