Kim Clijsters planned to stop playing tennis following the 2007 season to start a family, but she didn't even make it that far, deciding to walk away in May that year.
Former No. 1 Martina Hingis had already caused one surprise when she decided to hang up her racket at only 22 years old, only to come back and stun the sporting world even more when, in November 2007, she announced that she was stopping again because of a positive test for cocaine.
Belgium's Justine Henin caught the tennis world completely off guard when she abruptly quit while holding the No. 1 ranking in May 2008, just days before she was supposed to defend her French Open crown. The player who sent her into retirement, Dinara Safina, then got to No. 1 herself, only to take a break with back injuries in 2011 and never return. Henin did return in 2010 only to leave without much warning again in 2011.
A year ago, Marion Bartoli came from nowhere to win Wimbledon. Then, also from nowhere, came a retirement announced just a few weeks later. Now, Friday, two-time Grand Slam champion Li Na joined these sudden-stoppage players, citing recurring knee injuries.
It has been a roller-coaster year for the 32-year-old Li, who had admitted considering retirement following the French Open in 2013 only to rebound and reach the quarterfinals of Wimbledon and the semifinals of the US Open, as well as taking Serena Williams to three sets at the WTA Championships. She picked back right up at the beginning of this year, winning her second Grand Slam title at the Australian Open, reaching No. 2 and going 20-3 to start the season before withdrawing with a knee injury at Stuttgart.
The physical problems seemed to interrupt her momentum, and then came a psychological blow. Carlos Rodriguez, Henin's former coach and the person who had returned Li to Grand Slam contention following her 2011 French Open victory, was leaving after nearly two years of working together.
"She cried," Rodriguez said to Belgian press about telling Li, explaining that he was under contract to a Chinese academy that had allowed him to go on tour with Li but now wanted him to be based in China.
Li has not played since, withdrawing from Flushing Meadows with the knee injury.
And, of course, the unexpected WTA retirement often precedes an unexpected comeback, such as Hingis, Clijsters in 2009 and Henin. But they were all in their 20s, so the older Li is less likely to follow their path.
Li's retirement should not underestimate the impact she had on the game. As the most successful player from China, she became the world's second-highest earning athlete, helped popularize the game in the nation of 2 billion people and accelerated the WTA's move into Southeast Asia. There are now 17 events in the region, including the WTA Championships, which move to Singapore this year.
It is also thought that she will inspire a generation of Chinese players to build on this new tournament infrastructure; contemporary Peng Shuai, 28, has already stepped up by reaching the US Open semifinals earlier this month.
Though it will not be noted in the record books, Li's humor won fans as much as any of her on-court achievements. Her speech at the Australian Open, highlighted with quips about her "lucky" husband, was as acclaimed as her winning the title itself.
If that proves to be her victory lap, it was a good line to exit on.