BEIJING (Reuters) - Argentina beat Nigeria 1-0 under a strength-sapping midday sun to take a second consecutive Olympic football gold on Saturday and avenge defeat by the Africans in the 1996 final.

Mountain bikers also struggled with the heat, as well as mud and boulders, on the penultimate day of the Games, with riders from France and Germany picking up golds but many others failing to finish the challenging course.

Canoeists and kayakers pushed themselves to the limits of endurance in a flood of sprint finals on Lake Shunyi, a Spanish silver medallist was sick on the podium and a Ukrainian bronze medal winner fainted on the finish line.

To prevent similar scenes, the football final was stopped for water breaks in the 30th and 70th minutes.

The noon kick-off was the price paid for holding the final in the magnificent Bird's Nest stadium but it sapped the life out of the game, and both coaches said it had affected their players.

"We don't make the rules," said Nigerian coach Samson Siasia. "They said we had to play the game at 12 o'clock, which I didn't think was a very good idea."

Despite boasting the skills of Lionel Messi and orchestration of Juan Roman Riquelme, the Argentines never found their stylish best. But they were well worth the win thanks to a delightful second-half chip from striker Angel Di Maria.

With temperatures over 40 degrees Celsius (104 Fahrenheit) in the sun, Nigeria mounted plenty of late pressure but were unable to reproduce the drama of 12 years ago when a last-gasp goal gave them victory despite Argentina's protests of offside.

The start time also underscored the second-tier place the "beautiful game" has at the Olympics.

Governing body FIFA, wary of creating a rival to its own World Cup, restricts the Olympic tournament to under 23 year olds, with each team permitted three over-age players.

The Bird's Nest will transform itself back into an athletics stadium for a final evening of sport on Saturday, when Ethiopian long-distance runner Kenenisa Bekele will attempt to be the first man since 1980 to win both 5,000 and 10,000 metres.

Victory would complete a clean sweep of the track distance golds for Ethiopia, matching Jamaica's extraordinary sprint performance. Tirunesh Dibaba became the first woman to win both distance golds when she won the 5,000m on Friday night.

In the usual east African running rivalry, Bekele's toughest competition in the 5,000m comes from a trio of Kenyans and their former compatriot Bernard Lagat, now running for the U.S. team.

UNSTOPPABLE CHINA

There was a sour note on Saturday when the International Olympic Committee said it had disqualified Ukrainian weightlifter Igor Razoronov after he tested positive for drugs.

Day 15 is one of the busiest of the Games, with 32 medals up for grabs. In the mountain biking, Julien Absalon of France won his second straight Olympic gold and Germany's Sabine Spitz came home with time to wave a flag as she crossed the finish line.

Russia overtook Britain to claim third place in the medals table, reaching 19 golds with wins in canoeing and synchronised swimming, underlining their dominance of that sport.

Nevertheless their haul is way below their 27 Athens golds.

Russia's fall is more than matched by China's rise, and the hosts are now assured top spot in the medals table, with 48 golds to 31 for the United States.

Zhou Luxin is favourite to give them another and complete a clean sweep of all eight diving medals in the 10m platform.

"Everyone who has come before me has done their duty, so I have to make an extra effort," he said.

China's position on top of the medal table was widely predicted, given their strength at events like diving and table-tennis, but the size of the gap has surprised many.

With one fifth of the world's population to choose from, Chinese authorities have poured billions into a Soviet-style training system geared to maximising medals. Their new sporting superpower status reflects their emerging global economic might.

As the Olympic flag is lowered at Sunday's closing ceremony and passed to 2012 hosts Britain, China will feel the $43 billion investment on the Games was money well spent.

Despite a buildup dominated by talk of pollution and human rights, attention during the Games has focused on China's dazzling venues and the scintillating sport inside them.

The individual standouts have been Jamaica's Usain "Lightning" Bolt and American swimmer Michael Phelps, who won eight gold medals to become the most successful athlete in the history of the Games.

Despite some small protests over Tibet, a couple of militant attacks in west China around the start of the Games, and the murder of a U.S. tourist, little has distracted from the sport.

A record number of nations, 85, have taken medals, including, for the first time, conflict-riven Afghanistan whose taekwondo bronze winner Rohullah Nikpai was rewarded with a house.

(Reporting by Beijing Olympics bureau; Editing by Alison Williams)