Tyson Gay won the 100m at the US trials as 19-year-old Trayvon Bromell became the first teenager to qualify for an American World Championships team.
Gay, 32, clocked 9.87 seconds and will return to the Worlds in Beijing in August for the first time since 2009.
Bromell ran 9.84 in the heats, making him the 10th-fastest man in history, and was second in the final with 9.96.
Diamond League champion Justin Gatlin has a bye for the 100m in Beijing but will still race in the 200m trials.
Gatlin, 33, has run 2015's fastest 100m and 200m - 9.75 and 19.68 seconds respectively - while double-Olympic champion Usain Bolt has posted 10.12 in the 100m so far in 2015 after coming back from injury.
Bromell, who ran a wind-assisted 9.76 seconds in his semi-final, had the advantage over Gay early in the trials final but the 2007 world champion powered through in the final 30m and won by almost a tenth of a second.
"That kid - he's tough," Gay said of Bromell. "He got out good. It was just one of those 10 years of experience, dig-down moments I had to get him. It felt good though.
"This win right here may be my most important win. I feel like it was the toughest."
Mike Rodgers claimed third in 9.97 seconds and also qualified for Beijing.
In Thursday's heats, 10 Americans ran the 100m in under 10 seconds. Though all of those sub-10 performances were wind assisted - half of them beyond the permitted two metres per second - the obvious depth in American sprinting will fuel their hopes of threatening Jamaica's recent dominance.
Meanwhile at the Jamaican trials in Kingston, former world-record holder Asafa Powell surged to the 100m title but 2011 world champion Yohan Blake failed to make the final.
Powell, 32, pulled away from the field in the last 20m to equal his season best of 9.84.
However Blake, 25, was sixth in Powell's semi-final in a time of 10.36. The 2012 Olympic 100 and 200m silver medallist won the world title in 2011 when Bolt was disqualified for a false start.
Bolt withdrew from the trials on Thursday.