Tyson Gay's preparations for the World Championships continued with an impressive win in the men's 100m at the Diamond League meeting in Lausanne.
Gay, the 2007 world champion, has now clocked the three fastest times of the year after running 9.79 seconds to beat Jamaica's Asafa Powell (9.88).
Britain's former Olympic champion Christine Ohuruogu only managed to finish fourth in the women's 400m.
"Tyson Gay has always wondered if he will aggravate his groin if he runs the 200m as well as the 100m. But there doesn't look to be any signs of problems. He looks confident and ready.
"It wasn't a good run by Christine Ohuruogu's own standards and she couldn't put her finger on why she ran so badly. But she won't lose heart because she is still in good form. Moscow is the highlight of her year and that's where she wants to do well - collecting medals as she always does."
Shara Proctor took third in the women's long jump with a season's best 6.92m.
But attention was focused on American sprinter Gay, who appears to be back to his best form after struggling with injuries in recent years.
He did not make a blistering start but surged ahead of Powell, who recorded a season's best, in the final 40m.
It was another impressive performance from the 30-year-old Gay, following his world-leading 9.75 at the recent US Championships, and maintains the pressure on Usain Bolt ahead of August's World Championships in Moscow.
Jamaican Bolt, 26, decided not to compete in Switzerland and will instead run over 200m in Paris on Saturday.
"I'm feeling good and I think I have a good chance against anybody," Gay told BBC Sport.
His fellow American Francena McCorory, 24, clinched victory in Ohuruogu's event after holding off a late challenge from 2011 world champion Amantle Montsho to win in 50.36.
Ohuruogu, who won in Birmingham on Sunday, made a slow start as she was chased down by Botswana's Montsho, 30.
The 29-year-old Briton produced a trademark burst down the final straight but it was not enough to catch McCorory, Montsho and Jamaica's Novlene Williams-Mills.
Fastest 100m times of 2013
Tyson Gay (US) - 9.75
Tyson Gay (US) - 9.79
Tyson Gay (US) - 9.86
Justin Gatlin (US) - 9.89
Justin Gatlin (US) - 9.91
Justin Gatlin (US) - 9.94
Usain Bolt (Jam) - 9.94
Usain Bolt (Jam) - 9.95
"It wasn't a good race, it was a bit scrappy," said Ohuruogu, who finished in 51.03. "It wasn't good from the start - I didn't have enough commitment going into the back straight. I don't have any excuses."
But her compatriot Proctor, 24, impressed as she finished behind Nigeria's Blessing Okagbare (6.98m) and American Brittney Reese (6.96m) in the long jump.
Bohdan Bondarenko eased to victory in the men's high jump - but was unsuccessful in his bid to break Cuban legend Javier Sotomayor's world record of 2.45m, set in 1993.
The 23-year-old Ukrainian failed in his three attempts at 2.46m, while Britain's Olympic bronze medallist Robbie Grabarz (2.24m) finished eighth.
Former Olympic champion Dawn Harper-Nelson ran 12.53 - a season's best - to edge a tight women's 100m hurdles from three of her American team-mates.
A tenth of a second separated Harper-Nelson, Kellie Wells, Lolo Jones and Nia Ali, while Britain's Tiffany Porter (12.65) clocked her best time of 2013 to finish fifth.
Australia's reigning world and Olympic champion Sally Pearson, continuing her comeback from a hamstring injury, was seventh in 12.69.
In the men's 200m, Briton James Ellington (20.46) took fourth, behind winner Churandy Martina of the Netherlands (20.01).