Perth, Australia  - Serena Williams is booked for a series of three exhibitions this week in the United States, but that fact has not kept the world number two from withdrawing as defending champion at next month's Hopman Cup.

Williams and James Blake had been due to defend their title at the eight-nation mixed team competition starting January 3.

But Williams, claiming a hamstring injury, said she would be unable to participate. Her place in the team of five-time champion US will be taken by world number 50 Meghann Shaughnessy.

Shaughnessy also rode to the rescue for Hopman organizers a year ago, arriving in time to play the first match for the US when Williams failed to show up in Australia on time.

Shaughnessy owns six singles and 15 doubles titles on the WTA.

Unpredictable Marat Safin has discarded his plan to retire from tennis after another poor season and instead will play at the Burswood Dome to compete for Russian alongside younger sister Dinara Safina, who made a huge breakthrough to third in the world on the WTA in 2008.

Teams:

US (Meghann Shaughnessy, James Blake) Russia (Marat Safin, Dinara Safina) France (Alize Cornet, Gilles Simon) Australia (Casey Dellacqua, Lleyton Hewitt) Italy (Simone Bolelli, Flavia Pennetta) Germany (Sabine Lisicki, Nicolas Kiefer) Slovak Republic (Dominika Cibulkova, Dominik Hrbaty) Taiwan (Su-Wei Hsieh, Lu Yen-Hsun) (dpa)