Answer by taking a quote from the following article:

Born in Dusseldorf, West Germany, Kaymer turned professional at age 20 in 2005 and is a member of the European Tour. He has won eleven tournaments on the tour including four in 2010 to win for the first time the Race to Dubai, formerly the Order of Merit. Among those wins was the PGA Championship in the United States, which made him only the second German (after Bernhard Langer) to win a major championship. He also won the WGC-HSBC Champions to become the tenth player to win both a major title and a World Golf Championship event.

Entering the 2011 season, Kaymer turned down a chance to become a full PGA Tour member; he had gained exempt status with his win in the PGA Championship. He stated he would concentrate on the European Tour for 2011, but would play several U.S. events as well.  In January, Kaymer claimed his third Abu Dhabi HSBC Golf Championship title in four years and displaced Tiger Woods as number two in the world rankings.  After his runner-up finish at the 2011 WGC-Accenture Match Play Championship, Kaymer overtook Lee Westwood as the number one golfer in the world, making him only the second German (after Bernhard Langer) to be the top-ranked golfer in the world. At the time he was the second youngest to reach world number one behind Tiger Woods, soon surpassed by Rory McIlroy in March 2012, who gained the top ranking at age 22. In April, he relinquished his number one ranking after eight weeks to Westwood, who won the Indonesian Masters.  After reaching the number one ranking, Kaymer decided to undergo a swing change to be able to move the ball both ways. Frustrated with his disappointing results at the Masters, Kaymer looked to better shape a draw, a shot he thought he needed to be able to contend at Augusta. Kaymer missed the cut at the Masters for the fourth time in 2011 and later admitted that changing his swing for Augusta was a "big mistake." The rest of 2011 was relatively inconsistent for Kaymer.  In November 2011, Kaymer won his first WGC title at the WGC-HSBC Champions event in Shanghai, China. He entered the final round trailing Fredrik Jacobson by five strokes, then shot a final round 9-under 63 to take the title by three strokes from Jacobson. After parring his opening six holes, Kaymer birdied nine of the remaining twelve, with four straight birdies at the start of the back nine. This was the biggest comeback win ever in the history of the WGC events, and the lowest final round by a WGC winner, topping a 64 set by Hunter Mahan in 2010. Kaymer became the tenth player to have won both a major and a WGC event, and the win took him back to world number four.

What happened in 2011?
Entering the 2011 season, Kaymer turned down a chance to become a full PGA Tour member;