The Waterford star was 0.09 seconds behind Estonian Rasmus Magi's successful mark of 48.93, going through as a fastest loser.

American Rai Benjamin won in 48.52 seconds, ahead of home prospect Abderrahman Samba of Qatar in 48.72, with Takatoshi Abe of Japan also eliminated in third with 48.97.

Barr's previous best of the campaign had been 49.11, ranking him fourth on times for the year going into this third and last semi.

European record holder and defending champion Karsten Warholm of Norway was fastest in the round, winning his semi-final in 48.28 seconds, and is looking good to successfully retain his crown.

Alison dos Santos of Brazil won the opening semi in 48.35, a new lifetime best.

Mark English had the slowest season's best going into his heat of the men's 800m in Doha, and that ranking came to pass as he finished seventh in his heat of the two-lap event.

Having needed a late invite to compete after narrowly missing the qualifying time for Doha, the three-times European medallist from Letterkenny, clocked 1:47.25 in a heat won by Ferguson Rotich of Kenya in 1:45.98, where the top five went through.

The Donegal man was ranked 32nd of the 46 overall, and missed out on a fastest loser's spot by three-quarters-of-a-second.

Brendan Boyce represents Ireland late tonight in the longest event of the week, the men's 50k walk, which comes in the aftermath of a heavily-criticised women's marathon last night, run off post-midnight in temperatures of 35 Celsius.

28 out of the starting field of 68 failed to finish the race, which was won in the slowest World Championships time ever - Ruth Chepngetich claiming it in 2 hours, 32 minutes, 43 seconds.

Tonight's walk should take approximately four hours to complete.