Though Williams has too seldom been a steady wind in her long, intermittently phenomenal career, the tour’s continuity problems were hardly all about her in 2011. Kim Clijsters, a sentimental favorite with global reach, looked set to keep gathering momentum after beating Li to win the Australian Open in January. But injuries kept her from playing another Grand Slam tournament.

So much for building interest and rivalries around her, but at least Clijsters plans to give it another whirl in 2012. Her Belgian compatriot and rival Justine Henin, a purist’s delight who was the best women’s player of the 2000s not named Williams, retired abruptly for the second time in January after doctors determined that her damaged right elbow was too fragile for the powerful forces at work in the modern game.

Venus Williams, who, like Henin, has won seven Grand Slam singles titles, also appears to be in danger of forced retirement. She, like her younger sister, played little in 2011 and eventually revealed at the U.S. Open that she has been struggling with Sjogren’s syndrome, an autoimmune disorder that she says has sapped her energy.

For now, Venus Williams — age 31 and ranked 104 — will attempt to play on. So will Serena, her prospective Olympic doubles partner, who remains, despite all her own troubles, the gold standard for 21st century women’s tennis.

Serena Williams still managed to finish 12th in the rankings in 2011 and to reach the U.S. Open final, where she lost both her temper and the match against a suddenly all-business Sam Stosur. But Serena’s 22-3 match record did give her the tour’s best winning percentage, 88 percent, ahead of Kvitova’s 83.6 percent.

In a season when four different women won the Grand Slam singles titles, Kvitova was rightfully player of the year. She won Wimbledon in style, dominating the final against the much more experienced Maria Sharapova, whose serving issues continue to hold her back. Kvitova later won the year-end championships and then the Fed Cup with the Czech Republic. She is an imposing if hardly haughty left-hander who can generate crushing power and acute angles with her serve, forehand and two-handed backhand. But she has relatively limited mobility in an era that would seem to require great athleticism and has had disquietingly poor results of late on hard courts, which remain the tour’s base surface.