NEW YORK (Reuters) - Serena Williams, who has dropped to 172nd in the world after being sidelined for almost a year with injury and illness, has opted to use an injury-protected ranking to gain entry into the U.S. Open, the United States Tennis Association (USTA) said on Wednesday.

The American lost in the fourth round at Wimbledon last month after being out of action for nearly a year -- during which she suffered a foot injury and life-threatening blood clots.

Clijsters, who will be seeking a third-successive U.S. Open crown, has had her season disrupted by injury since beating China's Li Na to win the Australian Open in January.

She injured her ankle while dancing at her cousin's wedding, resulting in a two-month lay-off. She returned to play in the French Open but was knocked out in the second round by Dutch world number 114 Arantxa Rus.

The U.S. Open runs from August 29-September 11 in New York and will include 97 of the top 100 women players.

(Reporting by Frank Pingue in Toronto)