Serena Williams is finishing 2015 with a flourish despite not playing an official match since September.

Williams reached another milestone in her historic career with her 150th straight week as the No. 1 women's player, according to wtatennis.com.

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Williams joins Steffi Graf, who had 186 consecutive weeks at No. 1, and Martina Navratilova, whose streak at No. 1 lasted 156 weeks, as the only players to remain No. 1 in the world for at least 150 consecutive weeks.

Serena recently earned Sports Illustrated's "Sportsperson of the Year" award for 2015 after winning the year's first three Grand Slams and reaching the semifinals of the U.S. Open a stunning upset loss to Roberta Vinci.

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Both she and coach Patrick Mouratoglou have said the injuries -- mostly to her knees -- took their toll during the season's final months, requiring her to finish her 2015 prematurely. She did not play in any events beyond the U.S. Open.

Barring injury in 2016, Williams seems poised to break Graf's record of 186 straight weeks at No. 1, which Serena would surpass around next year's U.S. Open.

Since regaining the No. 1 ranking on Feb. 18, 2013, Williams has a 171-13 record, including 53-3 in 2015. She has won 22 of the 39 tournaments she's played in that span, including six of the 11 Grand Slam events.

Williams is still two years away from eclipsing Graf's record of 377 overall weeks at No. 1. Williams is at 273, 104 behind Graf, or exactly two years. Navratilova is second with 332, so Serena won't take second place until 2017 at the earliest.

Just more milestones for Williams to aim for as she continues her quest to become the greatest women's tennis player of all time.

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