Tyson Gay lost on his return to action after a drugs ban as he came second to Justin Gatlin in the 100m at the Diamond League meeting in Lausanne.
Gatlin, the 2004 Olympic champion, won in 9.80 seconds from fellow American Gay, 31, who ran 9.93secs on his comeback from a 12-month suspension.
"I'm happy to be back," said 2007 world champion Gay.
"Of course, I was worried about the reception I'd get. I feel very welcomed by my fellow athletes and fans."
A third American, Mike Rodgers, came third in 9.98secs, meaning the 100m podium in Switzerland was filled by men who have served drugs bans.
Gay's ban, which was reduced from two years by the United States Anti-Doping Agency because of his cooperation, ended on 23 June.
"Knowing what I've been through, I've been doing it for nine years professionally, I had to go back to old habits and focusing on myself. With a few more races, my time will come back down," added Gay.
Gatlin set a personal best of 9.79secs when he won bronze behind Jamaican pair Usain Bolt and Yohan Blake at the London Olympics.
|Tyson Gay facts|
|Personal bests: 100m - 9.69 secs; 200m - 19.58 secs|
|Medals: Olympics: 4x100m relay - silver (2012); World Championships: 100m - gold (2007), silver (2009), 200m - gold (2007), 4x100m relay - gold (2007)|
He was beaten again by Bolt in last year's World Championships, taking silver in 9.85secs.
Gatlin, 32, said: "I'm really happy to have put together a season best and I'm really on track to be running fast.
"Tonight was an easy run and I expected more pressure from the guys, but my body feels great and I'm injury free, so I'm running fast."
Olympic champion Kirani James ran the fifth fastest time in history in the 400m, finishing in a Commonwealth record 43.74secs.
In the absence of Bolt, who is recovering from a leg injury, fellow Jamaican Yohan Blake finished sixth in the 200m in 20.48secs.
Panama's Alonso Edward, the 2009 world silver medallist, won in 19.84secs.
Trinidad and Tobago's Michelle-Lee Ahye beat Murielle Ahour from the Ivory Coast in a photo finish in the women's 100m after both ran 10.98secs.
Earlier, Great Britain's James Dasaolu recorded 10.03secs to win the 100m 'B' race to improve his chances of selection for the European Championships squad.
Greg Rutherford, the Olympic long jump champion, was beaten into second by American Jeff Henderson, who jumped 8.31m to the Briton's 8.19m.
Britain's Lynsey Sharp ran a personal best of one minute 59.67 seconds to finish fifth in the 800m, while compatriots Rhys Williams were eighth in the 400m hurdles (50.30secs) and Steve Lewis pole vault (5.47m) respectively.