Tyson Gay ran the year's fastest 100m in 9.79 seconds in Florida on Saturday to send a warning to rival Usain Bolt.
Gay, 28, unbeaten in 100m last season, has said he needs to improve to keep up with the triple Olympic champion.
The 2007 world champion bettered the 9.89 run by Jamaican training partner Steve Mullings in May.
Mullings later had the chance to upstage Gay at the Diamond League meeting in Oregon but was just outside the time,
Gay also clocked the fastest time of 2010 with 9.78 and beat Bolt, who is also the world champion, when the two met in Stockholm last year.
The world's top two sprinters are not scheduled to go head-to-head until the World Championships in Daegu, South Korea in August with Gay insisting that Bolt, who holds the world record 100m time of 9.58, remains the dominant athlete.
Few would argue with Gay's views, but the American seems to have overcome the injuries he has struggled with since his double sprint glory in Osaka four years ago, and his current form is impressive.
Gay clocked a stunning 14.51 on the on 15 May in the 150m race, and followed that with an electric sprint in the United States on Saturday.
It may have only been a low-key event in a heat in the National Training Center Sprint Series in Clermont, Florida, but the performance should, at least, cause some concern for Bolt.
The Jamaican Bolt recently after a 10-month absence with victory at the Diamond League meeting in Rome.
But it was a rather lacklustre display by the colourful Jamaican's own standards, winning in 9.91.
He continues his Diamond League season in Oslo in Norway on 9 June, while Gay next runs the 100m at the Diamond League meeting in New York on Saturday 11 June.