Which one of the Grand Slam tournaments is still played on the same surface as it was when it was first played?

Wimbledon is the only Grand Slam tournament that has maintained its original grass surface since it began in 1877. The Australian Open was originally played on grass but switched to hard courts (Rebound Ace, then Plexicushion). The French Open has always been on clay but started later than Wimbledon. The US Open began on grass, then switched to clay for three years, before settling on hard courts.