World number one Lleyton Hewitt is undergoing medical tests in Perth after suffering a serious bout of fever at the Hopman Cup today.
Tournament officials would not say whether he had gone to hospital or not but did confirm that tests were being carried out away from the tournament site.
"Lleyton is having tests to see if the fever is being caused by a virus," tournament director Paul McNamee said.
Hewitt suffered from the fever on Tuesday night and said that he had been feeling unwell for the previous couple of days.
But, despite the illness, Hewitt played in Australia's Hopman Cup victory over Argentina at the Burswood Dome on Wednesday, beating Mariano Zabaleta in the singles and teaming up with Alicia Molik to win the decisive mixed doubles.