Justin Gatlin shocked Olympic champion Usain Bolt to win the 100m in 9.94 seconds in Rome.
Bolt was quickest out of the blocks at the Diamond League meeting but American Gatlin powered past, one-hundredth of a second ahead of the Jamaican.
Britain's Shara Proctor finished third in the women's long jump while Hannah England was fourth in the 1500m.
Fellow Briton Dai Greene, the 2011 400m hurdles world champion, was fifth in his first race of the season.
Gatlin, an Olympic champion in 2004 who went on to serve a four-year doping ban, maintained his unbeaten start to the season over 100m, recording his third Diamond League success after victories Doha and Eugene by the narrowest of margins.
Bolt, who suffered a hamstring injury last month and was making his debut in the competition this season, jumped out of the blocks but was blowing hard by halfway as Gatlin began to impose himself on the race.
However, he had to dip his head on the line to beat the world record holder, with France's Jimmy Vicaut third in 10.02 seconds.
"Bolt just wasn't very good. He had a
very good start and the quickest
reaction time, but there was nothing
"He was blowing hard to run under
10 seconds and normally that's a
stroll for him.
"He won't like being beaten by Gatlin.
Gatlin is a controversial figure and a
lot of people don't think he should be
competing, but he is, and he's looking
good for the World Championships.
"But don't write off Bolt - he always
raises his game at the major
Bolt, who ran a season's best 9.95, told BBC Sport: "I got the perfect start that I wanted, then about five steps in I stumbled a bit and that kinda threw my game off.
"My legs didn't feel like my legs coming into the straight. I guess I need to do more strength work. I think I just need time to get it back together but the fact I got a good start threw me off. At least I got under 10 seconds."
There was another surprise in the women's 200m where Olympic champion Allyson Felix was well beaten into second by Ivory Coast sprinter Murielle Ahoure.
American Felix was trailing Ahoure coming off the bend and was unable to catch the Ivorian who ran a national record of 22.36 seconds.
Proctor produced the highest finish by a Briton, with third in a competitive long jump, finishing just eight centimetres behind London 2012 champion Brittney Reese, with Janay Deloach Soukop splitting the pair.
In the 1500m, England sprinted clear of the bunch to claim fourth in a time of four minutes, 03.91 seconds behind Sweden's Abeba Aregawi, who won in 4:00.23 from Ethiopia's Genzebe Dibaba and America's Jennie Simpson.
Greene was left trailing by the fast-starting Johnny Dutch and Javier Culson in the 400m hurdles and was unable to catch the duo as they went clear in the finishing straight.
American Dutch held on to win in 48.31 with Puerto Rico's Culson second and Mamadou Hanne third - Greene was fifth in 48.56.
Phillips Idowu finished a distant seventh in the triple jump - his opening leap of 16.44m proving to be his best as American Christian Taylor won with 17.08m