Djokovic won 6-3 7-5 in a largely commanding performance.
The 29-year-old, who has now won seven titles this year, has beaten the Japanese on nine successive occasions.
Djokovic, who was beaten in the third round at Wimbledon by Sam Querrey, will represent Serbia at the Rio Olympics, reports the BBC.
“I don’t need to explain that every athlete dreams of being a part of the Olympic Games,” he said. “I’m competing in singles and doubles.
“Hopefully I’ll get at least one medal.”
