A history of upsets and a top-order shake-up show that at the Olympic tennis tournament, only one thing can be expected: The unexpected.

Since returning to the Olympic fold in 1988, the sport has produced a role-call of surprise champions, including Marc Rosset and Nicolas Massu, who have left the big names trailing in their wake.

In 2004 Roger Federer, then at the start of his four-and-half-year stint as world No 1, was sent crashing out in tears in round two by the 74th-ranked Tomas Berdych.

"Just a really bad day," lamented the Swiss.

But Federer, who joined Andy Roddick, Tim Henman and Juan Carlos Ferrero on the sidelines, was merely following in a grand tradition of high-profile losers.

China's Zheng Jie attends a tennis practice session in Beijing August 5, 2008. [Agencies]

Miloslav Mecir swept past Wimbledon champion Stefan Edberg on his way to the 1988 title, one of the highlights of his career.

Four years later Switzerland's Rosset became the lowest-ranked men's winner when he took gold at a world rating of 44.

Only one of the top 11 seeds had made it as far as the Barcelona quarterfinals, with Edberg and Thomas Muster among the first-round victims.

Andre Agassi became the first star player to win the men's title in 1996, when he took advantage of the absence of Pete Sampras, Boris Becker and Michael Chang.

Yevgeny Kafelnikov had been locked in a dispiriting slump until he won at Sydney 2000, and in Athens unheralded Chilean Massu pulled off an unthinkable shock by claiming the singles and pairs double within two days.

Massu battled through a five-set final high on excitement but low on star quality against America's Mardy Fish before being greeted by thousands of fans and Chile's president on his return home.

The women's competition has historically been smoother for the top players, with Steffi Graf, Jennifer Capriati, Lindsay Davenport, Venus Williams and Justine Henin composing a distinguished list of winners.

However, the No 1 ranking has changed hands three times since Henin's retirement in May, indicating vast potential for surprises.

Maria Sharapova's withdrawal through injury has made the competition even less predictable, with China's Wimbledon semifinalist Zheng Jie among the dark horses.

Among the men, Federer can still become the first No 1 to take gold, although he will yield the top ranking to Nadal on August 18 - the day after the final.

Federer's indifferent form, and Rafael Nadal's antipathy for hard courts, point to yet more upsets.

In the doubles, China is hoping for a repeat of Li Ting and Sun Tiantian's surprise win of 2004, when they beat vaunted pairings led by Venus Williams and Conchita Martinez en route to the title.

Sun will team up with Peng Shuai while Zheng is paired with Yan Zi as China aims to repeat the historic feat in front of its fervent home fans.

The Olympics, sandwiched between Wimbledon and the US Open, has traditionally taken a back seat to Grand Slams. But Beijing is hosting most of the big players including 17 of the men's and women's combined top 10s.