An outmatched U.S. team paved the way for the first visit by an American president to the Chinese mainland. — In Tokyo last weekend, the U.S. teams competed in the lightning-fast game of table tennis, an Olympic sport since 1988. It was also the sport that, 50 years ago, launched a breakthrough in U.S.-China relations known as “Ping-Pong diplomacy.” In April 1971, at the table tennis championships in Nagoya, Japan…