DEBRECEN, Hungary -- Alexei Nemov's challenge for three gold medals at the 36th world championships got off to a disastrous start on Thursday.

But while the Russian all-round Olympic winner faltered, the Chinese men topped the qualifiers in two of the six men's apparatus disciplines.

Nemov, one of the most successful gymnasts in history with 11 Olympic medals to his name, came unstuck on both the horizontal and parallel bars.

The Sydney all-round champion lost his rhythm in the middle of a swing and landed awkwardly on the parallel bar during his first event.

Nemov acknowledged his disappointing score of 8.300, which immediately eliminated him from contention, with a bemused smile and helpless shrug, Reuters reported.

The defending Olympics horizontal bars champion, who had a dreadful outing in Ghent twelve months ago when he failed to reach any of the finals, next performed a daring routine incorporating six release and catch moves.

His creativity was spoilt by a stumble on landing but a score of 9.462 left him facing an anxious wait before scraping through to the semifinals in 15th place.