LONDON - Chinese swimmer Sun Yang broke his own world record to make it a golden double at the London Olympics on Saturday, while US superstar Michael Phelps drew the curtain on his magnificent career by claiming his 18th Olympic gold medal - the 22nd medal of his career.

As the Games reached its halfway mark, the United States is locked in a tight battle with China in the race for No. 1 in the medals. America is now on top with 54 medals, of which 26 are gold. China has 25 gold and 53 medals in total.

Sun won in a time of 14 minutes 31.02 seconds, eight and a half seconds ahead of his nearest rival and 3.12 seconds faster than the previous best he set in Shanghai last year. The 20-year old became China's first ever male Olympic swimming champion after winning the men's 400m freestyle last week.

"You don't understand how much I wanted this gold medal," said Sun, who beat the water in joy and broke down in tears after he saw his results from the screenboard. "I really, really wanted it, and today I made it."

But it was Phelps who stole the most limelight Saturday night at the Acquatics Center.

In his farewell Olympic race, Phelps swam his favorite butterfly leg to help the US to gold in the 4x100m medley relay.

"I've been able to do everything that I wanted," said the 27-year-old Phelps, a four-time Olympian who has announced to retire after the London Games.

The Americans also won the women's 4x100m medley relay in world record time, underlining their depth of talent in the pool.

Ranomi Kromowidjojo of the Netherlands posted an Olympic record of 24.05 seconds to take the women's 50m freestyle title, adding to her 100m gold.

The Americans also claimed two gold medals in tennis, with Serena Willimas taking the women's singles title and Mike and Bob Bryan winning the men's doubles.

Williams needed just over an hour to crush Russia's Maria Sharapova 6-0, 6-1 to complete a "golden slam" of all four majors and Olympics singles titles.

Two days after its two female doubles players were disqualified for alleged trying to lose on purpose, China cemented its dominance in badminton when Zhao Yunlei and Tian Qing beat Japanese Fujii Mizuki and Kakiiwa Reika 21-10, 25-23 in the women's doubles final.

Li Xuerui, who didn't secure a spot in China's Olympic squad until the last moment, was crowned the women's singles champion after she pipped more heralded teammate Wang Yihan 21-15, 21-13, 21-17 in an all-Chinese affair.

Chen Ding became the first Chinese to win an Olympic gold medal in men's race walk. The 19-year-old pulled away from the lead pack shortly after the 18-kilometer mark to finish the 20-kilometer race in 1 hour, 18 minutes, 45 seconds. Barrondo took silver and Wang Zhen of China claimed bronze.

In fencing, China took the women's epee team gold after outclassing South Korea 39-25 in the final.

On the busiest day of the Games so far, with 25 gold medals up for grabs in 11 sports, Britain moved into third place with 29 medals, of which 14 are gold, in the medals table after a hual of six gold in rowing, cycling and athletics.

In front of 80,000 vocal spectators at the Olympic Stadium, Team GB's iconic athlete Jessica Ennis lived up to expectations with a comfortable triumph in the heptathlon. She amassed 6,955 points, the third highest in Olympic history, to give the host nation its first gold in the athletics action.

"The crowd helped me and I can't believe I've done it," said Ennis. "I'm going to savor the moment. I've had great support, although I have been under a huge amount of pressure."

Greg Rutherford then followed suit by taking the men's long jump title with a best leap of 8.31 meters, before Mo Farah upset double-Olympic champion Kenenisa Bekele for the men's 10,000m gold.

British cyclists Joanna Rowsell, Dani King and Laura Trott also smashed the women's team pursuit world record - for the third time this week - to claim their fourth gold medal out of five so far in the Velodrome, leaving them on course to match their Beijing record of seven.

Also at the Olympic stadium, Jamaica's Shelly-Ann Fraser-Pryce clocked 10.75 seconds to retain her Olympic 100m title.

Men's defending 100m champion Bolt clocked 10.09 seconds in his first round heat for the blue riband event of the Games while Jamaican compatriot Blake timed 10 seconds.