By John O'Brien
SINGAPORE (Reuters) - When Roberta Vinci completed a stunning semi-final upset of Serena Williams at the U.S. Open in September, the Italian not only ended her opponent's hopes of a calendar Grand Slam but inadvertently deprived the WTA Finals of its star attraction.
Fatigued by the strain of getting to within two victories of claiming all four majors in one season, a demotivated Williams hung up her racket for the year and decided against taking aim at a fourth straight title in the elite eight-woman event.
The world number one's absence from the Singapore tournament starting on Sunday is a blow for its organizers and an injury cloud hangs over three of the players who have made the journey to southeast Asia.
Nevertheless, a high-class octet are hoping to end the year in style.
Flavia Pennetta benefited most from Vinci's triumph, beating her friend and compatriot in the final to claim a maiden grand slam title at the age of 33 and secure a berth in the singles portion of the season-ending tournament for the first time.
Pennetta's appearance will also be her last as she announced her plans to retire from tennis at the end of this season during the victory ceremony in New York. The likeable Italian would be a popular champion should she overcome her seven rivals next week.
"I don't exactly know what my plans are as I need to first finish here," Pennetta told reporters on Saturday. "I am really focused on my job, (I will) do what I have to do, finish this week and enjoy it a little bit as well."
The Italian limped out of her most recent tournament in Moscow earlier this week and the highest-ranked players in Singapore, Simona Halep and Maria Sharapova, have also suffered troubled build-ups to the season finale.
World number two Halep took a break from tennis after injuring her ankle at the start of the month, while third-ranked Sharapova has not completed a match since losing to Williams at Wimbledon, the Russian struck down with leg and arm problems.
"If I can stay healthy and work my way through the matches no matter who I'm playing, in this particular situation I have to worry about myself a little bit more than who I'm playing against," a wary Sharapova said.
As the top-ranked players plan to adopt a safety-first approach, the rest of the field will feel they have a chance to join a prestigious list of WTA Finals champions with a good run in Singapore.
Garbine Muguruza is the form horse in the field after the 22-year-old backed up her Wimbledon final loss to Williams with a maiden premier event title in Beijing this month but the fourth-ranked Spaniard played down her chances of victory.
"I have played really well this month and hopefully I can continue to do so, but I don't know if I can be regarded as one of the favorites," she said. "The first time, sometimes it's hard to deal with so let's just see what happens."
Of the others, double Wimbledon champion Petra Kvitova will be gunning for a second WTA Finals victory to go with her 2011 triumph but the big-hitting Czech can be prone to erratic play and will need to maintain her focus.
Round-robin play starts on Sunday when Halep opens proceedings against Pennetta at the Singapore Indoor Stadium before Sharapova takes on Agnieszka Radwanska in the night match.
(Editing by Clare Fallon)