Are there figurative English proverbs (or idioms) to mean an expert (or likely winner) makes a great mistake?

The latest news that Serena Williams lost the semi-final round of U.S. Open to an unseeded Italian player, Roberta Vinci, whom Williams had never lost in the past reminded me of Japanese proverb, 'Jouzu-no tekara mizu ga ochiru,' of which literal translation is ‘Water leaks even from expert's hands,’ meaning even a perfect expert makes a mistake, or loses chances, which can be never retreived like shed water.

We have several proverbs in the same token such as 'Kobo mo fudeno ayamachi' – Even Kobo (774 -835), a Japanese Buddhist saint known for a master of calligraphy makes a mistakes of stroke, and 'Sarumo kikara ochiru' meaning “even a monkey (who should be a master of tree climbing and hopping around branches) falls from a tree.”

Are there English proverbs or idioms that figuratively describe that an expert or perfect player makes a great mistake sometimes?

There are a couple of classical references: Homer nods, or Even Jove nods (formerly well-known, as in this Kipling quote; but nobody seems certain whether there is a Latin original, or somebody just 'upgraded' the reference).

"She's only human" is a common phrase used to remind one that no matter how great the person's achievements, they are, after all, only a human being with the occasional same set of flaws and frailties as the rest of our species.

I suggest the following sayings which, though not figurative, are close in meaning to what you are referring to:

We all make mistakes:

• used for telling someone not to worry about something that they have done wrong.

(www.macmillandictionary.com)

and nobody's perfect:

• Used when someone's mistakes or flaws are acknowledged, to remind that everyone else makes mistakes and has flaws.

(Wiktionary)