Double Olympic champion Mo Farah has switched from the 10,000m to the 5,000m for his opening Diamond League race of the season in Eugene.Farah, who won 5,000m and 10,000m gold at London 2012, changed events after suffering with a stomach virus.It means he will avoid facing world record holder Kenenisa Bekele over the longer distance.Christine Ohuruogu, Robbie Grabarz and Greg Rutherford are among the other Britons competing at the event.David Rudisha, who bettered his own world record to win 800m gold at last summer’s Olympics, has been forced to pull out with a knee injury.Farah now races on Saturday instead of Friday and will face his training partner Galen Rupp, who took 10,000m silver in London, 5,000m bronze medallist Thomas Longosiwa of Kenya and former world champion Bernard Lagat.The women’s 400m also boasts a strong field comprising six of the top eight in the world. They include all three medal winners at London 2012: champion Sanya Richards-Ross, silver medallist Ohuruogu and DeeDee Trotter of the USA.Rutherford, the Olympic long jump champion, will face a difficult job if he is to win his first Diamond League event of the season.He faces the last three Olympic gold medallists: Dwight Phillips, who won gold in 2004, and Irving Saladino, who won the first ever Olympic medal for Panama when he was crowned champion in 2008.Kirani James, the current world and Olympic champion in the 400m, will also race at the Prefontaine Classic, looking to continue the incredible form which has seen him become world champion at youth, junior and senior levels and Olympic champion in the last four years.The 20-year-old will face LaShawn Merritt, who won gold in Beijing, and Luguelin Santos and Lalonde Gordon who finished second and third last summer.