These dictionary entries may help to explain the confusion here:
American Heritage Dictionary
hard court - n. A tennis court with a hard surface, such as asphalt, rather than grass. Courts with a clay surface are usually considered hard courts outside of the United States.
http://www.answers.com/topic/hardcourthard court - DEF – For the United States: A cement, asphalt or similar paving for a tennis court. For Australia and European countries: A clay court.
http://www.btb.termiumplus.gc.ca/tpv2alpha/alpha-eng.html?la...It is thus even possible to find this:
tennis sur terre battue nm. hard-court tennis
http://dictionary.reverso.net/french-english/tennis sur terr...However, Flushing Meadows is a US hard court and I don't think anyone who follows tennis in the UK would refer to Roland Garros as anything other than a clay court.
See also the official Roland Garros site:
L'US Open
Le Decoturf (un revêtement dur) succède à la terre battue.
http://www.rolandgarros.com/fr_FR/about/grandslam/index.htmlUS Open
Decoturf hard courts also replaced the clay.
http://www.rolandgarros.com/en_FR/about/grandslam/index.html