The 25-year-old from New Jersey smashed her own best time in the 200m breaststroke at the Aquatics Centre, winning in 2:19.59 and becoming the first woman ever to swim the event faster than 2:20.
And as part of the American 4x100m medley relay team she collected a second winner’s medal after the team – also featuring the dazzling talents of Missy Franklin, Dana Vollmer and Alison Schmitt – finished in 3:52.05, almost two seconds ahead of Australia. She now has six Olympic medals and an astonishing five world records.
Soni was born in 1987 and is of Hungarian descent. Originally a gymnast, she took up swimming at the age of 10 so she wouldn’t have to wait for her older sister Rita to finish classes at her club. Of such happy accidents are Olympic champions occasionally made.
Known to her family as Reb, she first came to prominence in competition at the 2004 US Olympic team trials, where she raced in the 100m and 200m breaststroke but finished in 15th and 11th places respectively.
But a year later, as a student at the University of Southern California, she claimed her first international medals, winning silver over the same distances and gold in the 4x100m medley at the Summer Universiade in Turkey.
The following year she stepped up a gear, finishing a respectable fourth in the 200m breaststroke at the FINA World Championships. Her progress was all the more impressive given that she suffered from an irregular heartbeat at the time – a health problem that interfered with her training, but was ultimately resolved.
She had to settle for fourth place again in the 100m breaststroke at the US trials ahead of the 2008 Beijing Games – but when one swimmer withdrew and another missed the entry deadline she got her break and travelled to China to compete.
Soni didn’t disappoint. As well as taking silver in the 4x100 medley and the 100m breaststroke behind world-record-holder Leisel Jones, she caused a major upset in the 200m, beating the Australian’s record to win gold in 2:20.22.
The habit of breaking records continued in 2009 at the World Aquatics Championships where she won the breaststroke in 1:04.84. Further best times came in the 100m and 200m breaststroke and 4x100m medley relay at the Duel in the Pool contests in the UK in 2009 and the US in 2011.
And a year before the 2012 Games she cemented her status as a star swimmer with a trio of gold medals in her favoured 100m and 200m events.
Soni is not particularly tall for an elite swimmer, being the smallest of the US women’s relay team and some five inches shorter than Missy Franklin. However, her core strength, gained from a training regime that includes yoga and pilates, is immense.
Her inimitable breaststroke technique – which even she struggles to explain – is what separates her from her rivals. Much discussed, the unique style features an abbreviated leg kick allied to perfectly timed, rapid arm sweeps.
As team USA coach Sean Hutchison has said. ‘I don’t think she’s necessarily faster than everybody else. She just slows down less than everybody else. The timing and the way her arms and legs works is perfect.’
Her idiosyncratic style has made her, quite suddenly, one of the most dominant and consistent swimmers in international competition. Having proved herself in London, she is a favourite to win more gold medals and set new world records in Rio in 2016.