World champion Dai Greene dispelled any doubts about his form and fitness with an impressive personal best at the Diamond League meeting in Paris, but a seriously fast 5,000m race showed just how tough a challenge Mo Farah will face at London 2012.
Greene clocked 47.84 seconds, just 0.02secs outside Kriss Akabusi's 20-year-old British record, though it was only good enough for second place behind Javier Culson.
The Puerto Rican, who took silver behind the Welshman at the World Championships last year but is now firmly the world No 1, won in a world-leading time of 47.78s to further enhance his claims for Olympic gold in a month's time, while Greene's run was a welcome return to form following a slow start to the season caused by a knee operation he had over the winter and a bout of illness.
He was closing fast on Culson in the final stages, suggesting he will only get quicker as the summer progresses.
"PB tonight. Feels good to be back in business, all coming together at the right time. Ready for the big one." Greene said on Twitter.
Farah was not running in the French capital, but the 5,000m had no fewer than 10 athletes surpass his previously world-leading time of 12:56.98.
Ethiopia's Dejen Gebremeskel won in 12:46.81 to become the fifth fastest of all time over the distance and record the quickest time since 2005.
His fellow countryman Hagos Gebrhiwet was second in a world junior record of 12:47.53 as the first six home all went surpassed Farah's greatest recorded performance.
World and Olympic champion Kenenisa Bekele, also of Ethiopia, ran a season's best 12:55.79, but that only secured ninth, meaning he is unlikely to run over the distance at the Olympics.
In the 100m, Tyson Gay, back after injury, beat fellow American and former drug cheat Justin Gatlin to claim victory in 9.99s.
Gatlin is the third fastest man in the world this year with 9.80s, but he could not hold off Gay's challenge as the latter powered through in the second half of the race to nick it on the line. Gatlin, who served a four-year doping ban, was second in 10.03s as home favourite Christophe Lemaitre finished third with 10.08s.
David Rudisha destroyed the field in the 800m, winning by over four seconds in a world-leading 1:41.54.
The race very quickly turned into one between the imperious Kenyan and the clock as he finished 0.53s outside his own world record.
Australia's Sally Pearson looks another contender for gold in London after winning the 100m hurdles in a world-leading 12.40s, with Britain's Tiffany Porter finishing third.
Christine Ohuruogu ran a season's best 50.59s to finish fourth in the 400m as world champion Amantle Montsho won in 49.77s, while British record holder Shara Proctor had to settle for second in the long jump in 6.65m, with Russia's Yelena Sokolova winning in 6.70m.
Steve Lewis was fourth in the pole vault, Lisa Dobriskey eighth in the 1500m and Barbara Parker eighth in the 3,000m steeplechase.