Lleyton Hewitt and Andre Agassi have abruptly pulled out of the $US2.45 million ($A4.05 million) Monte Carlo Masters tennis tournament starting this weekend.

Australian prodigy Hewitt, ranked No.1 in the tournament and American veteran star Agassi, ranked No.2, had been the event's biggest names.

"Agassi gave no official reason as to why he is pulling out of the tournament," Matthew Rapp, ATP's vice president of Communications, said by telephone. "But he did say he regretted missing the event."

Hewitt pulled out because the tournament was "not on his schedule", Rapp added.

Tournament organisers offered no further explanation for the sudden changes.

Agassi and Hewitt joined a list of absentees that has dented the top half of the draw.

Others forfeiting include Roger Federer, who was ranked in fourth place in the tournament, and Russian star Marat Safin, a former US Open winner, who was ranked seventh.

Federer, a star of Switzerland's recent Davis Cup win over France, was suffering from a fever, while Safin had cited an ankle injury for his non-participation, an ATP official said earlier Friday.

France's Sebastien Grosjean, who strained his thigh in France's Davis Cup loss to Switzerland, and England's Tim Henman, suffering from a recurring shoulder injury, had dropped out earlier this week.

The top 52 ATP players, ranked six weeks before the tournament starts, are automatically entered into the main draw.

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