(CNN) -- Injuries continue to strike down the planet's top football players ahead of this month's World Cup in South Africa, with Dutch star Arjen Robben's participation in doubt and key Nigeria midfielder Jon Obi Mikel forced to withdraw.





Robben, whose inspired form this season took German club Bayern Munich to the final of the Champions League, has not joined his teammates in flying to Africa after suffering a hamstring injury on Saturday.





He scored two goals after coming on as a second-half substitute in a 6-1 friendly romp over Hungary in Amsterdam, but needs a scan on Sunday after hurting himself trying to execute a fancy backheel pass.





"I would rather lose this match and have Arjen stay fit," Netherlands coach Bert van Marwijk said on Dutch Web site www.vi.nl. "He felt a sharp pain. That does not bode well. But I do not lose hope."





Robben had returned to action after missing Tuesday's 4-1 international friendly rout over Ghana in Rotterdam on Tuesday.





Earlier on Saturday, Nigeria officials reported that Mikel has decided he has not recovered sufficiently from knee surgery, meaning he joins teammates Michael Ballack (Germany), Michael Essien (Ghana) and Jose Bosingwa (Portugal) from his English club Chelsea in missing the tournament.





The 23-year-old Mikel missed the end of Chelsea's season, which culminated in a league and cup double.





"We have dropped Mikel from the World Cup squad after he told the team he did not want to put his career at risk as his knee injury has yet to fully heal after a recent surgery," Nigeria team official Emmanuel Attah said in quotes carried by the Chelsea Web site.





Another Chelsea player, Ivory Coast striker Didier Drogba, may also be sidelined for the month-long event after suffering an elbow injury against Japan on Friday.





He had a "successful" operation in Switzerland on Saturday, according to Ivory Coast officials, but they could not yet say if he would be fit to take part.





Mikel followed England captain Rio Ferdinand in being denied a chance to play on soccer's biggest stage, with the defender suffering a knee injury on Friday.





He has been replaced in the 23-man squad by Tottenham defender Michael Dawson, with Liverpool midfielder Steven Gerrard taking over as skipper and Chelsea's Frank Lampard named as his deputy by coach Fabio Capello.





World champions Italy also have an injury problem, with key midfielder Andrea Pirlo suffering a calf injury against Mexico on Thursday.





The 31-year-old has been ruled out of Saturday night's final friendly against Switzerland, and is battling to be fit for the Azzurri's Group F opener game against Paraguay on June 14.





Marcello Lippi's team came from behind to draw 1-1, with striker Fabio Quagliarella's 15th-minute goal canceling out Gokhan Inler's opener.





Slovakia defender Martin Skrtel also suffered an injury blow on Saturday when he had to go off in the 16th minute of the 3-0 friendly win over Costa Rica after damaging his ankle.





The center-back, who plays for English club Liverpool, had previously played only once since breaking a bone in his foot in February. He is expected to be fit for the World Cup after resting his injury.





In Saturday's other friendlies, the United States beat fellow qualifiers Australia 3-1 with two goals from Edson Buddle and another from fellow forward Herculez Gomez, who came on as a substitute.





The 29-year-old Buddle, starting in place of the injured Jozi Altidore, netted his first goals for his country as he continued the form that has him at the top of the Major League Soccer goalscoring charts this season.





Midfielder Tim Cahill leveled for Australia in the 19th minute.





Hosts South Africa notched another morale-boosting win by beating fellow qualifiers Denmark 1-0 in Atteridgeville, as striker Katlego Mphela netted his fourth goal in three matches in the 76th minute.





Coach Carlos Alberto Parreira's team will kick off the tournament against Mexico in Johannesburg's new Soccer City stadium on Friday.





Ghana won 1-0 against Latvia in England with
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