NEW YORK (CNN/Money) - Nike Inc. said Thursday it has signed an endorsement contract with tennis star Serena Williams, but the world's largest sports apparel maker refused to disclose the amount of the deal.

A person familiar with the deal, however, told CNN/Money that the agreement is a five-year contract that would include royalties and performance bonuses for winning Grand Slam tournaments or reclaiming the title of world's highest-ranked player. The deal can also be extended beyond the deadline, he added.

But the amount of the deal is far below than $60 million speculated earlier this week, said the person, who spoke on condition of anonymity.

Three years ago, Nike (NKE: Research, Estimates) lost out to Reebok, which signed Serena's older sister, Venus, to a $38 million, five-year deal, the biggest for a woman athlete.

Even if Serena hit all the marks in the contract with Nike, the person told CNN/Money that the total amount she will receive will still fall short of Venus' deal with Reebok.

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"Without question, Serena is a world-class athlete, and we are very excited that she will be joining the team of Nike athletes," said Dave Mingey, spokesman for Nike.

The younger of the two Williams sisters will start wearing Nike attire beginning Jan. 1. Serena, 22, had received $12 million over five years from Puma, but her Puma contract expired early this year.

Serena has won five of the last seven Grand Slam tournaments, taking the No .1 seat for about a year until being sidelined because of a knee surgery on Aug. 1.

But she is expected to participate in the Australian Open early next year.