Swim prodigy Michael Phelps spearheads the United States quest for pool dominance at London Olympics as the team is ready for challenges from worldwide including China who expects a "breakthrough" to confirm new status among elites.

Though Phelps won't replicate his glittering eight-gold haul in the 2008 Games as he withdrew from the 200m freestyle, it is still exciting to imagine what the 14-time Olympic champion might accomplish in London.

Phelps scratches 200 free from London agenda.

Phelps, who seemed to be back in shape from a career dip after Beijing Olympics, is whetting appetites for a dazzling display in seven events at London Olympics: the 100m and 200m butterfly, 200m and 400m individual medleys and three relays.

The 27-year-old will have a chance to claim a third straight Olympic title in four events - the 100m and 200m butterfly and the 200m and 400m individual medley. No male swimmer has ever won the same Olympic event at three straight Games.

Besides Phelps, the US squad features a bevy of world and Olympic champions, such as Ryan Lochte, world record holder of 200m individual medley, and Rebecca Soni, 200m breaststroke Olympic champion.

Lochte, who last year emerged from Phelps's shadow with five titles at the World Championships in Shanghai, said he felt capable of matching Phelps's historic tally of eight golds in one Olympic Games.

Another US rising star Missy Franklin, whose three gold, one silver and one bronze at the Shanghai World Championships saw her named FINA's female swimmer of the year 2011, will seek her time of dominance on Olympic stage.

The Americans, with starry 214 gold medals in the history of Olympic swimming, will dominate again, but face increasing challenges from a fleet of superb swimmers worldwide including ambitious Chinese.

The Chinese squad is brewing a "big breakthrough" in London, said Vice-President of the China Swimming Association Shang Xiutang, adding the goal set for Chinese swimming in London is to better results of the Beijing Games and snatch gold in men's events.

China retained one gold, three silver and two bronze medals in Beijing four years ago, with favorite Zhang Lin becoming the first male Olympic medalist in history. However, Zhang was edged out of the London squad due to a career slump that led to his failure in national qualification.

World champion Sun Yang, who smashed Grant Hackett's 10-year-old 1,500m record at last year's Shanghai World Championships and has a clutch of world's best times in recent two years, is China's biggest gold favorite.

Sun Yang leads China's charge at medal in London.

Besides Sun's overwhelming reign in the 1,500m freestyle, the 20-year-old also targets at the 200m and 400m freestyle gold with a possible duel with South Korea's Olympic champion Park Tae-Hwan.

China's 51-strong team also boasts 200m butterfly Olympic champion Liu Zige, world champion Jiao Liuyang, 50m backstroke world title holder Zhao Jing, and bronze medalist of Shanghai World Championships Wu Peng, who beat Phelps in the 200m butterfly at the US Charlotte UltraSwim Grand Prix meet in May this year.

Liu will be joined by Jiao in the women's 200m butterfly to ensure a medal for China.

Britain's 400m and 800m freestyle Olympic champion Rebecca Adlington may lead the European charge, while Japan's four-time Olympic champion Kosuke Kitajima, with a dazzling medals haul under his belt, carries Japan's gold hopes and is seeking a straight men's 100m and 200m breaststroke double in London.

Australia will look for signs of a renaissance after slipping to fourth at Rome Championships in 2009 and third at Shanghai Championships in 2011. The traditional powerhouse has high hopes for triple Olympic champion Stephanie Rice, who ranks world top in women's 200m individual medley.

The swimming events are slated for July 28 to Aug 4 at London Olympics.