The American Allyson Felix was made to fight all the way before winning the 400 metres in last night's first Diamond League meeting of the season in Doha.
Felix made a quick start and looked in control of the race before Botswana's Amantle Montsho surged on to the triple world 200m champion's shoulder around halfway.
However, Felix was able to respond and held on at the line to record a world-leading time of 50.33 seconds, with Montsho just 0.08sec behind.
Felix, who won 21 of her 22 races last year, has yet to decide whether she will double up over 200m and 400m at this year's World Championships in Daegu, but admitted: "It's definitely something I'm considering."
There was more success for America in the women's 100m hurdles, with Kellie Wells leading a 1-2-3 from Danielle Carruthers and Lolo Jones.
Britain's Michael Rimmer produced an impressive performance to finish second in the 800m to Kenya's Asbel Kiprop.