CAPE TOWN, South Africa -- Corrie Sanders, the South African southpaw who knocked out Wladimir Klitschko in one of the great upsets in heavyweight boxing, has died after being shot by robbers at a restaurant during a family celebration.

The former WBO and WBU champion was 46.

Sanders was shot in the hand and stomach at a family member's 21st birthday party at the restaurant in Brits, in South Africa's North West province, on Saturday night, police said. He died in a hospital in the capital city of Pretoria in the early hours of Sunday morning.

Police Brigadier Thulani Ngubane said three armed men entered the restaurant with the apparent intention to rob it and shot randomly, hitting Sanders. No other injuries were reported. The robbers took a cellphone and a bag from customers, Ngubane said.

No arrests had been made but a murder investigation has been opened.

Renowned for his hand speed, the 6-foot-4 Sanders was one of South Africa's most successful and popular fighters after a nearly 20-year professional career.

He retired in 2008 with a 42-4 record, with 31 knockouts. But he was most remembered for the shocking second-round knockout of Klitschko in Germany in 2003 that earned him the WBO title and respect across the world as a wily, fast and clever fighter and powerful puncher.

It is one of only three losses for the Ukrainian and current WBO champion. Sanders agreed to the fight on short notice and was a 40-1 underdog. The result was voted as the upset of the year by Ring Magazine.

He then lost to Wladimir's older brother Vitali for the vacant WBC title the following year, Vitali's first fight for the title he still holds. Sanders had earlier relinquished his WBU crown in a seventh-round stoppage by Hasim Rahman after being ahead in their fight in 2000.

Born in Pretoria, Sanders began his professional boxing career in 1989 with a first-round TKO of King Kong Dyubele. Eighteen of the fast-starting Sanders' 31 career knockouts came in the first round.

Sanders hung up his gloves in 2008 after being knocked out by Osborne Machimana for the South African heavyweight title – in the first round.

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    HANNOVER, GERMANY - MARCH 8: Corrie Sanders of South Africa celerbrates his victory over Wladimir Klitschko of Germany during the WBO heavyweight fight at The Preussag Arena on March 8 2003 in Hannover, Germany. (Photo by Stuart Franklin/Getty images)

  • Corrie Sanders celerbrates

    HANNOVER, GERMANY - MARCH 8: Corrie Sanders of South Africa celerbrates his victory over Wladimir Klitschko of Germany during the WBO heavyweight fight at The Preussag Arena on March 8 2003 in Hannover, Germany. (Photo by Stuart Franklin/Getty images)

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    HANNOVER, GERMANY - MARCH 8: Corrie Sanders of South Africa and Wladimir Klitschko of Germany tie up during the WBO heavyweight fight at The Preussag Arena on March 8 2003 in Hannover, Germany. (Photo by Stuart Franklin/Getty images)

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    HANNOVER, GERMANY - MARCH 8: Corrie Sanders of South Africa and Wladimir Klitschko of Germany during the WBO heavy weight fight at The Preussag Arena on March 8 2003 in Hannover, Germany. (Photo by Stuart Franklin/Getty images)

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  • Corrie Sanders of South Africa is hittin

    SOELDEN, AUSTRIA: Corrie Sanders of South Africa is hitting Russia's Alexei Varakin during an international boxing Eurofight in Soelden, Austria, 14 December 2004. Sanders won by k.o.. AFP/ Dominic Ebenbichler (Photo credit should read DOMINIC EBENBICHLER/AFP/Getty Images)

  • Corrie Sanders of South Africa is hittin

    SOELDEN, AUSTRIA: Corrie Sanders of South Africa is hitting Russia's Alexei Varakin during an international boxing Eurofight in Soelden, Austria, 14 December 2004. Sanders won by k.o.. AFP/ Dominic Ebenbichler (Photo credit should read DOMINIC EBENBICHLER/AFP/Getty Images)

  • FILE This Wednesday April 14, 2004 file photo shows former WBO heavyweight champion Corrie Sanders of South Africa, right, answering questions during a news conference , at Steele Boxer in the Marina Del Rey section of Los Angeles. Corrie Sanders, the South African southpaw who knocked out Wladimir Klitschko in one of the great upsets in heavyweight boxing, died Sunday Sept. 23, 2012 after being shot by robbers at a restaurant during a family celebration on Saturday night. The former WBO and WBU champion was 46. He died in a hospital in the capital city Pretoria in the early hours of Sunday morning. (AP Photo/Nick Ut, File)