Australian middle-distance superstar Stewart McSweyn has been forced to withdraw from the world indoor championships as he struggles with the after-effects of COVID-19.
McSweyn was unable to finish a 5000m race in Melbourne on Tuesday night due to breathing problems and will now be sent for respiratory and cardiac tests.
"We need to get an exact diagnosis of what it is," McSweyn's coach Nic Bideau told AAP.
"We think it's down to COVID because this only started when he caught COVID (in January) but you never know.
"He's fine at cruising pace but when he goes at a really hard pace it's different.
"He was well clear in front last night, he only had one kilometre to go and he couldn't make it.
"He wasn't in any danger of losing but he just couldn't run any further."
McSweyn recently had his COVID booster shot and entered the race in suburban Box Hill in an attempt to prove his fitness for the world indoor championships in Belgrade beginning on March 18.
"That was the acid test," said Bideau.
"I said to Stew 'if you can't get the qualifying time for the 5000 in that race then I won't send you'.
"He's a top-level athlete and I'm not sending him over there to run indoors with the world championships and the Commonwealth Games coming up later this year."
In the absence of the national 1500m record holder, the 15-strong team for the world indoors in the Serbian capital will be headlined by in-form high jumper Eleanor Patterson and Tokyo Olympics decathlon bronze medallist Ash Moloney.
Patterson has made a flying start to the year as the overall winner of the World Indoor Tour in the women's high jump.
The 25-year-old old broke the Australian indoor record with a clearance of 1.99m in Birmingham last month and will be among the gold-medal favourites.
Moloney will contest the heptathlon in Belgrade in his first indoor competition, where he is expected to shine in the 60m sprint, 60m hurdles and the field events.
Even without McSweyn, Australia will be well represented in the middle-distance events by a host of world-class performers including Ollie Hoare (1500m), Linden Hall (1500m), Jess Hull (1500m and 3000m) and Catrona Bisset (800m).
Sprint star Rohan Browning experienced hamstring tendon irritation after racing in Adelaide last month and has decided to give the world indoors a miss, mindful of a busy year ahead.
Tokyo Olympics high jump silver medallist Nicola McDermott also made herself unavailable for selection.
AUSTRALIAN WORLD INDOOR ATHLETICS CHAMPIONSHIP TEAM
Women: 800m Catriona Bisset. 1500m: Linden Hall, Jessica Hull. 3000m: Lauren Ryan, Jessica Hull. 60m Hurdles: Liz Clay. High jump: Eleanor Patterson. Pole vault: Nina Kennedy.
Men: 400m: Tom Willems. 800m: Charlie Hunter. 1500m: Ollie Hoare., Stewart McSweyn. 3000m: Matthew Ramsden. 60m hurdles: Nick Andrews, Chris Douglas. Pole vault: Kurtis Marschall. Heptathlon: Ash Moloney.
