Johannesburg - Sunette Viljoen struggled in poor, wet conditions, finishing fourth at the Diamond League athletics meeting in London on Saturday.
Viljoen, the top-ranked women's javelin thrower in the world this year, launched a 63.33 metres effort in the first round for her best attempt on the second and final day of the London Grand Prix.
With her next five throws, she managed only one more heave over the 60m mark.
Goldie Sayers of Great Britain made a statement, setting a British national record of 66.17m to win the event just two weeks before the start of the London Olympic Games.
World record holder Barbora Spotakova of the Czech Republic took second place with a best of 64.19m, and Vira Rebryk of the Ukraine was third with a 63.80m throw in the third round.
The only other South African who competed in the penultimate Diamond Leage meeting before the London Games, Khotso Mokoena, was third in the men's long jump on Friday's opening day.
Mokoena leaped 8.24m but he was narrowly edged by Australian Mitchell Watt (8.28m) and Chris Tomlinson (8.26m) of Great Britain.