American doubles stars Bob and Mike Bryan have clinched the No. 1 spot and qualified for the 9th time for a spot in the Barclays ATP World Tour Finals in London, the ATP said.

The 35-year-old twins won the doubles title at the Masters 1000 event in Cincinnati over the weekend, piling up enough points to make the year-end tournament.

"Never before has a team clinched year-end No. 1 this early in the season. Cincinnati was their 10th title of an outstanding 2013 season, which has seen them triumph at the Australian Open, Roland Garros and Wimbledon, making them the first team in tennis history to hold all four majors and the Olympic gold medal at the same time," the ATP said.

The brothers are heading to the U.S. Open, which starts on Aug. 26, with the goal of becoming the first doubles team in the Open Era to win the calendar Grand Slam.

"To clinch the year-end No. 1 ranking this early in the season is a dream come true," Bob Bryan said. "It's always our ultimate goal starting out each season and this will give us great confidence going into New York. There is still a lot of tennis left and we want to keep our foot on the gas."

The Barclays ATP World Tour Finals will take place Nov. 4-11 at London's O2 Arena. EFE