BEIJING (AP) -- The 2015 season is over for Serena Williams.

The No. 1-ranked Williams withdrew from her final two scheduled tournaments of the year on Thursday, saying she needs time to heal from a series of injuries and the heartbreak of coming up short in a bid for a calendar-year Grand Slam.

Williams hasn't played since losing to Roberta Vinci in the semifinals of the U.S. Open, where she came two matches short of becoming the first player to win all four majors in the same year since Steffi Graf in 1988.

''It's no secret I've played injured most of the year - whether it was my elbow, my knee, or, in the final moments after a certain match in Flushing, my heart,'' Williams said in a statement.

Williams pulled out of next week's China Open in Beijing and the season-ending WTA Finals in Singapore from Oct. 25-Nov. 1.

''I want to compete as well as I can as long as I can,'' she said. ''So I'm taking a proactive step and withdrawing from tournaments in Beijing and Singapore to properly address my health and take the time to heal.''

In 2015, Williams won the Australian Open, the French Open and Wimbledon.

Williams said she plans to get back on the court during the offseason and prepare for the Australian Open in January, where she will be going for her 22nd Grand Slam title. That would tie Steffi Graf for most in the Open era.

''This is a very difficult decision, but one ultimately made because of the love of the game,'' Williams said. ''I plan to return to practicing and participating in exhibition matches later this year. And when I do, l will focus and focus and focus so I can continue my journey in this beautiful game.''

Williams has already secured the year-end No. 1 ranking for the third straight year and fifth season overall.

Williams won the last three WTA Finals, which features the world's top eight players. She has won the tournament a total of five times.

''Serena's health has to be her No. 1 priority,'' WTA president Micky Lawler said in a statement Thursday. ''The risk of injury is a reality in the world of elite athletic performance. We will miss seeing Serena on court in both Beijing and Singapore and wish her a speedy recovery.''

Earlier this week, No. 3-ranked Maria Sharapova pulled out of the China Open because of a left forearm injury. She said she still hopes to play at the WTA Finals.