World number ones Roger Federer [Images] and Ana Ivanovic head a strong field at the Olympic tennis tournament in Beijing [Images] next month.

The International Tennis Federation (ITF), which released the entry lists for the men's and women's singles and doubles on Wednesday, said Beijing would witness the best lineup since tennis returned as a full medal sport in Seoul 1988.

Fifty-six players gained direct entry based on their singles rankings immediately after last month's French Open with 17 of the men's top 20 and 18 of the women's top 20 confirmed for the tournament which will run from Aug. 10-17.

Six men and women were handed ITF places in recognition of their records in the Davis Cup and Fed Cup despite not meeting the rankings criteria.

Swiss Federer, who stayed on course for a sixth consecutive Wimbledon title on Wednesday by reaching the semi-finals, is attempting to win his first Olympic medal after a surprise defeat in Athens four years ago by Czech Tomas Berdych.

French Open champion Ivanovic, defeated by China's Zheng Jie in the third round of Wimbledon last week, will be making her Olympic debut.

The top five in the men's rankings will all play in the singles with the only absentees from the top 20 being American Andy Roddick [Images] and Richard Gasquet of France [Images].

MASSU RETURNS

Spain's Fernando Verdasco is ineligible because he is the fifth qualified player from Spain.

In the women's singles Anna Chakvetadze [Images] of Russia [Images] and last year's Wimbledon runner-up Marion Bartoli are missing from the women's top 20.

Chile's Nicolas Massu, singles and doubles champion in 2004, has been handed one of the six ITF places because his world ranking has slipped to 131. Veteran Swede Jonas Bjorkman, who retires at the end of the season, has also been granted a place in the draw.

"I'm very happy. I'm always glad to represent my country," Massu said in an ITF statement. "I have great memories of four years ago."

Liezel Huber is in the U.S. team as a doubles player after competing for South Africa at the 2000 Olympics [Images]. She gained American citizenship in 2007.

Countries are allowed a maximum of six players in the Olympic tournament with no more than four in the singles. They are allowed two doubles pairs.

Tennis was a part of the first modern Olympic Games in 1896 but was withdrawn after the 1924 Paris Games. It returned as a demonstration event in the 1984 Los Angeles Games and became a full medal sport again in 1988.