By Brian Homewood
LONDON (Reuters) - Olympic champion Caster Semenya maintained her two-year unbeaten record over 800 meters as she eased through the first round at the World Championships on Thursday.
Her main challengers, Francine Niyonsaba, Margaret Wambui and Ajee Wilson, also cruised into Friday's semi-finals but defending champion Maryna Arzamasova was eliminated after finishing fourth in her heat.
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South African Semenya, last beaten at the World Championships semi-final in Beijing two years ago, was third going into the final straight and eased past Rose Mary Almanza and Natoya Goule to win in 2:01.33.
"I just want to keep winning. That's all I'm concentrating on. I'm feeling okay after the 1,500 meters, so I'm ready to go again," said Semenya, who won bronze in the 1,500 on Monday.
Wilson, who became the first American to run under 1.56 seconds when she set a national record in Monaco last month, comfortably won the first heat in 2:00.52.
"I'm definitely in the best shape of my life as my time in Monaco before the championships showed," Wilson said. "I've never run so well and so consistently during my career."
Burundi's Niyonsaba and Kenyan Wambui, silver and bronze medalists respectively in last year's Olympics, also won their heats.
Arzamasova led the fourth heat on the last lap but faltered badly at the finish to be pipped for the third automatic place by Halima Nakaayi. She was not among the six fastest losers.
Kenyan Eunice Sum, the 2013 world champion, withdrew due to illness.
(Editing by Ed Osmond)