Martina Navratilova of the Czech Republic is the tennis player who held the world No. 1 ranking in both singles and doubles simultaneously. She achieved this feat on September 10, 1984, when she was ranked No. 1 in both singles and doubles. Navratilova won a total of 18 Grand Slam singles titles and 31 Grand Slam doubles titles in her career, and she is widely regarded as one of the greatest tennis players of all time. She was known for her outstanding serve-and-volley game, her versatility on all surfaces, and her incredible fitness and endurance.