The US Open Series is the name given by the United States Tennis Association (USTA) to a series of North American professional tennis tournaments leading up to and including the US Open. It is part of the "North American hard court season". Emirates sponsored the series in the past, under a deal in place from 2012 to 2016. The series was initially organized in 2004 as a way to focus more attention on American tennis tournaments by getting more of them on domestic television. Until 2004, most summer North American tournaments were not on television, the exceptions being the prominent ATP Tour Masters 1000 events in Canada and Cincinnati. Since the inception of the series, Rafael Nadal is the only tennis player to win Canada, Cincinnati, and the US Open in a calendar year (2013), a feat referred to as the "Summer Slam" or the "North American Hardcourt Slam". Under the US Open's broadcast rights, ESPN held domestic rights to all US Open Series events from 2015 to 2019. The eight non-Masters tournaments receive about 50 hours of television combined – about two hours on each day of their final weekends, chiefly on ESPN2. The dual-gender Washington D.C. Citi Open (the only ATP 500-level tournament in North America), which had been a part of the series since its 2004 founding, withdrew from the series for its 2015 edition due to frustrations over this lack of coverage; that tournament sold its exclusive coverage to Tennis Channel. The tournament re-joined the US Open Series in 2019. Since 2017, Tennis Channel broadcasts the US Open Series. Bonus Challenge Players earn points for the US Open Series Bonus Challenge in order to win a bonus on top of the individual series tournaments' prize money, according to their results in these events. More points are awarded at some of the events, such as the ATP World Tour Masters 1000 and WTA Premier 5 events. The three male and three female players with the most points in the US Open Series Bonus Challenge earn a money purse. The amount depends on their US Open Series placement and US Open result. If both are won then the bonus is $1 million as of 2010.Lleyton Hewitt and Lindsay Davenport were the top point-getters in 2004, Andy Roddick and Kim Clijsters won in 2005, and Andy Roddick and Ana Ivanovic won in 2006. Defending US Open champions Roger Federer