The late withdrawals of Poland's rising star Jerzy Janowicz and Spanish veteran Tommy Robredo have thrown wide open Perth's Hopman Cup which starts Saturday. The Polish pairing of Janowicz and Agnieszka Radwanska were the top seeds for the A$1 million (HK$6.92 million) mixed teams tournament, but the world number 21 withdrew with a foot injury. I was excited to play the Hopman Cup but an injury to my arm has given me no choice but to withdraw from the event. I wish I could be there to play with Anabel and help defend the title for Spain Tommy Robredo He has been replaced by little known countryman Grzegorz Panfil, ranked 288th in the world, in the opening event of the Australian summer, which begins preparations for the year's first grand slam, the Australian Open. Meanwhile, the rejuvenated Robredo, a two-time winner in Perth, withdrew just 24 hours out from the event on Friday with an arm injury, and has been replaced by Daniel Munoz-De La Nava, ranked 194th in the world. Munoz-De La Nava is the 17th-ranked male Spaniard in world tennis, and will be partnered by defending champion Anabel Medina Garrigues, who won the event with Fernando Verdasco earlier this year. "I was excited to play the Hopman Cup but an injury to my arm has given me no choice but to withdraw from the event," Robredo said. "I wish I could be there to play with Anabel and help defend the title for Spain." The Hopman Cup has rarely been predictable in its 25-year history and the absence of Janowicz is a major blow to Radwanska's hopes of getting her hands on one of the unique diamond-encrusted tennis balls presented to the winners as Poland make their debut in the tournament. The world number five is likely to need to win all her singles matches in the group ties, which will not be easy against the likes of Italian Flavia Pennetta, Australia's Samantha Stosur and the promising Canadian Eugenie Bouchard. Then she and Panfil will need to thrive in the rare challenge of mixed doubles that makes the Hopman Cup such an unusual tournament. While Poland still nominally start the tournament as top seeds, bookmakers have the second-seeded United States pairing of Sloane Stephens and John Isner as favourites as they seek their country's seventh Hopman Cup title. Stephens, ranked 12th, emerged as a player of rare promise in Australia earlier this year, defeating Serena Williams en route to a semi-final appearance at the Australian Open. The towering Isner, best remembered for playing the longest ever grand slam match against Nicolas Mahut at Wimbledon in 2010, is ranked 14th in the world and is also a past winner at the Hopman, having teamed with Bethanie Mattek-Sands to take the title in 2011. Isner teamed with Venus Williams in Perth earlier this year, but was forced out of the tournament by a knee injury. Third seeds are the French pairing of Alize Cornet and world number 10 Jo-Wilfried Tsonga, both of whom boast Hopman Cup experience. Tsonga is the highest-ranked man in the event, but will still be playing in Abu Dhabi when the tournament kicks off, with France's first tie against the Czech Republic on Monday. Big-serving Milos Raonic, ranked one place below the Frenchman at 11, teams with Bouchard for Canada, while the host nation is represented by Stosur and the enigmatic Bernard Tomic, who beat Novak Djokovic in this event at the start of the year. Raonic said the Hopman Cup was a good way to prepare for the Australian Open, which begins on January 13. "I've always played well in these conditions where I've sort of had this patriotic pressure that I put on myself," he said. "I really enjoy these moments and to be able to do it in a more team atmosphere makes the week more enjoyable and it's a little bit of a change, which is nice to have." Pennetta and Andreas Seppi carry Italy's hopes, with 2012 winner Petra Kvitova and Radek Stepanek representing the Czech Republic. Group A features Poland, Canada, Italy and Australia, while Group B is the United States, France, Czech Republic and Spain, with the top two teams after the group matches meeting in the final on January 4.