Union Jack-wielding fans at London's Olympic Park had much to cheer about Saturday as the Games' host nation raked in six wins, bringing Great Britain's gold medal count to 14.

Only China and the United States have more.

Jessica Ennis earned the heptathlon gold, Greg Rutherford scored a long jump win and Mo Farah's sprint to victory made him the first British runner ever to do so in the 10,000 meter competition.

Meanwhile, the U.S. leads the gold medal count with 26 among its 54 total medals. China is second with 25 golds and 53 overall.

But history on Saturday also favored the leaders, as the most decorated Olympian of all time tallied another win with his American teammates in the 4 x 100-meter medley relay, handing Michael Phelps his 18th gold medal in the last race of his storied swimming career.

The U.S. win put an already unsurpassed Phelps at 22 total medals over three Olympic Games: 18 gold, two silver and two bronze. Japan placed second in Saturday's race, while Australia finished third.

In the last day of London's swimming competition, the American women won the women's 4x100-meter medley relay race, setting a new world record with a time of 3:52.05. Australia netted silver.

And Ranomi Kromowidjojo of the Netherlands won gold in the women's 50-meter freestyle, finishing in 24:05 seconds.

Sun Yang also added to China's growing gold tally with a win in the men's 1500-meter freestyle. Ryan Cochrane of Canada finished second, while Oussama Mellouli of Tunisia took bronze.

But on the hardwood Saturday, U.S. Olympic command seemed far less convincing.

Team USA squeaked past Lithuania in a preliminary round basketball contest, winning by just five points after trailing by two in the waning minutes of the fourth quarter -- the first scare for the favored American squad after a series of dominating performances that had bolstered an aura of invincibility.

Led by LeBron James and Kobe Bryant, the U.S. squad had overwhelmed France, Tunisia and Nigeria in prior rounds before nearly losing to the underdog team Saturday, which evoked comparisons to the 2004 Athens Games when Lithuania shocked the United States by pulling off a 94-90 victory.

"We had to make some big plays to win," said USA head coach Mike Krzyzewski . "So far in the tournament things have come easy to us, because we've been hitting so many shots."

James, the reigning NBA MVP from the Miami Heat, sunk nine of his 20 points in the last four minutes to help the American stars eek out a 99-94 win.

But their come-from-behind victory raises questions about how the team will fare against stiff competition, including Argentina, a 2-1 team that they're set to play on Monday.

Elsewhere in London, Serena Williams completed a "career Golden Slam" at the 2012 Games, adding for the United States an Olympic women's singles tennis gold medal to her four tennis major titles.

Williams, playing in her third Olympics, dominated Russia's Maria Sharapova 6-0, 6-1 to continue her 17-match winning streak. Sharapova was playing in her first Games.

Williams, who had previously twice won gold in doubles, claimed the singles gold on the same day twin American brothers Mike and Bob Bryan won the gold medal in men's doubles. Frenchmen Julien Benneteau and Richard Gasquet took bronze.

Williams will also seek a third gold in doubles with her sister, Venus Williams. The sisters clinched at least a silver when they defeated Maria Kirilenko and Nadia Petrova of Russia on Saturday.

Meanwhile, Great Britain won a gold medal in cycling and the world's fastest man, Jamaican sprinter Usain Bolt, got his 100-meter campaign under way on day eight of the Games.

Great Britain's women's team pursuit squad set a new world record in claiming the gold against the United States, which won silver. The British team finished in 3:14.051, some five seconds ahead of the Americans.

But all was not golden for the host nation, as its men's football team had its hopes dashed in dramatic fashion Saturday when South Korea beat them 5-4 on penalty kicks, ousting the Brits from the Games. Korea advanced to the semi-finals to face off against Brazil.

At the Olympic Stadium, Bolt won his heat in a time of 10.09 seconds after what he described as a poor start.

He faces a challenge from teammates Yohan Blake, who has stolen some of his thunder this year and won his heat in 10.0 seconds, and Asafa Powell, who also crossed the line first in his race.

It was also a good morning on the track for Team USA, with Justin Gatlin and Ryan Bailey getting off to a scorching start by both posting sub-10 second times, while teammate Tyson Gay also won his heat.