Monks didn't just invent beer and clocks, they also invented the game of tennis, probably in the 12th or 13th century, the evidence is unclear as the exact date. They strung up a rope and used their hands to bounce the ball off the monastery walls, over the rope.
Eventually the game was played indoors, as well. The use of their bare hands gave way to the use of gloves for protection, which became gloves with webbing between the fingers, to make it like a small net. Soon, a paddle was held instead, then webbing attached to handle became the forerunner of modern tennis racquets or tennis rackets.
The first games of tennis weren't even played with anything we would consider a ball today. Wads of cork, hair or wool would be gathered tightly into a round shape and secured with string, fabric of leather. Later years had "balls" stitched inside felt.
By the 16th century, the ancestor of tennis racquets was created by stringing sheep gut onto a wooden frame. The tennis court was standardized, but doesn't resemble the tennis court of modern times. The indoor courts were narrow, and players still bounced the balls off the wall. And the nest were two feet higher on each, rather than the standard height of modern tennis court nets.
Tennis became a popular game once French royalty learned it from the monks. It became so popular, in fact, that in at attempt to control the masses, the Pope and the King tried to ban the tennis. But the game's popularity spread to England, where Henry VII and VIII enjoyed the game so much, they had more courts built.
Tennis was played less and less frequently in the 18th century. Then Charles Goodyear invented a process of making rubber in 1850, and tennis players were interested in the new material enough to revive the sport a bit. Major Walter Wingfield patented tennis rackets and tennis equipment in 1874 that closely resembles our modern tennis equipment. Croquet courts were fairly easily adapted to tennis. Finally, a rectangular court and specific set of rules for playing tennis was standardized in 1877 when the first Wimbledon was held by the All England Club. The first US Open tournament was played in 1871. The games were singles only, and the players all male.
Tennis' rather strange scoring system is attributed to two different sources. One story says that in medieval times, the number 60 was considered a good and complete round number, must like 100 is considered today. So a score of 60 was considered a complete score, and the 4 scoring intervals in tennis were deemed 15, 30, 45 and 60 (though in modern times the 45 has become 40, it was 45 originally).
Other says that the tennis scoring system is related to another monk invention: the clock. A clock was used at the end of the court to keep score, by having the hand moved a quarter of an hour with each point.