The United States experienced highs and lows in Friday's Diamond League event in Monaco, with an injury scare for LaShawn Merritt but world-leading times for hurdler Aries Merritt and the men's 4x100m relay team.
LaShawn Merritt, the favourite for 400m gold at the London Games, pulled up holding his left thigh after 300 metres of his race, although he insisted that his premature withdrawal was purely a precaution.
"At 250 metres, as I started to move my leg up, I felt a little bit of discomfort," said a smiling Merritt, who explained that he had felt "cramp in my hamstring".
"The aim of the season is the Olympic Games, so I decided to stop. I'd hoped to do a good time here but I'll continue working to be ready for London."
Belgium's Jonathan Borlee was a surprise winner in 44.74sec, ahead of Grenada's Kirani James (44.76sec).
Merritt's countryman Wallace Spearmon also endured a dispiriting evening, as he trailed in third in the 200m behind Jamaica's Nickel Ashmeade (20.02sec) and Dutchman Churandy Martina (20.07sec).
Spearmon's time of 20.09sec was 0.44sec slower than his personal best of 19.65sec, which he set in 2006.
Disaster struck for the women's top 4x100m relay team, who were unable to finish their race after a mix-up during the final baton hand-over.
The 'Red' team, featuring Tiana Madison, Allyson Felix, Jeneba Tarmoh and Carmelita Jeter, had established a healthy lead but Tarmoh and Jeter were unable to transfer the baton at the last change-over.
"It was a communication problem," explained Tarmoh.
"We were in the right positions but I couldn't put the baton in her hand. It was only the second time that I was passing to Carmelita.
"I don't think it'll make a difference for the Olympic Games. We should keep the same four girls."
The USA's second-string relay squad, known as the 'Blue' team, nonetheless won the race, with Lauryn Williams, Alexandria Anderson, English Gardner and Kimberlyn Duncan coming home first in a time of 42.24sec.
There was much better news in the men's race, where Trell Kimmons, Justin Gatlin, Tyson Gay and Ryan Bailey romped to victory in 37.61sec -- the fastest time in the world this year.
"I felt good and it's good for the confidence," said Gay, who ran the third leg.
"It wasn't a perfect race but it was enough to win tonight. There wasn't a huge amount of people here but the fans were shouting and I heard lots of noise, so I didn't want to take any risks.
"At the current time, the third leg seems to be the best position for me."
Aries Merritt ran a world-leading time of 12.93sec in the 110m hurdles for the third time this year to confirm his credentials as a serious contender for the gold medal in London.
There were also year's best marks for Kenya's world and Olympic champion Asbel Kiprop in the men's 1500m (3min 28.88sec) and Germany's Silke Spiegelburg in the pole vault (4.82m).
Russian pole vault queen Yelena Isinbayeva endured a dismal night as she failed with three attempts to clear 4.70m, gifting victory to Spiegelburg and raising serious questions about her chances at the London Olympics.
"I was ready to jump but everything went differently to what was expected," said the two-time Olympic champion.
"I'm physically and mentally ready for the Games, of course, but as a human, I'm worried like any other competitor."
Nigeria's Blessing Okagbare triumphed in the women's 100m in a personal best time of 10.96sec, while Kenyan Conselus Kipruto celebrated a memorable victory in the 3,000m steeplechase (8min 03.49sec) at the age of just 17 years and 225 days.
The ninth leg of the Diamond League circuit was the last major meet before the Olympics begin on July 27.
US sprinter Tyson Gay (R) takes the baton from teammate Justin Gatlin in the men's 4x100m relay during the IAAF Diamond League Herculis meeting at the Stade Louis II in Monte Carlo, Monaco. The United States' men's 4x100m relay team set the fastest time in the world this year by prevailing in 37.61sec.
Russian Yelena Isinbaeva competes in the women's pole vault contest during the IAAF Diamond League Herculis meeting at the Stade Louis II in Monte Carlo, Monaco. Isinbayeva endured a dismal night as she failed with three attempts to clear 4.70m.