Li Na of China was the toughest player on the women's tour in the first two Grand Slam events of 2011.
First she made the final of last year's Australian Open, where she lost to Kim Clijsters, then Li went one better in winning the French Open several months later with a victory over defending champion Francesca Schiavone.
She has maintained her high ranking due in no small part to her successes at Melbourne Park and at Roland Garros. Since the title in Paris, however, Li has not been that effective on tour.
The Chinese star definitely skidded down the stretch of 2011, as she suffered losses to Monica Niculescu, Simona Halep and Petra Cetkovska. Those are three players you would expect a top 10 player to defeat.
Li is the defending champion at this week's WTA Sydney and is through the first round in that event. In fact, she actually looked strong in dismissing Ekaterina Makarova in straight sets 6-0, 6-3.
"Upping" her game counts for something, but good form is not a light switch and I think Li will fail to defend her Sydney title.
If she falters there and fails in the middle rounds of the Australian Open (or earlier) then it could be the beginning of the end of the current world No. 5.
She has the fifth seed at Melbourne Park, and that might be good for two or even three early-round wins, but come the fourth round and the quarters, Li will likely be facing players that won't let her bad form pass.
Li will probably stay in the top 10 with the post-Australian Open ranking updates, as her 2,000 ranking points for winning the French Open will remain intact until June.
She will be 30 years old in February, however, and her future in the upper echelon of women's tennis looks very uncertain.