Roger Federer became a Grand Slam champion again this year when he won his seventh Wimbledon title, but winning his 18th Grand Slam singles title will be incredibly difficult in 2013.
To his credit, Federer played well this season. He made the semifinals at the Australian and French Opens, won Wimbledon, and was upset in the quarterfinals at the US Open by Tomas Berdych. He also won the silver medal in men's singles at the 2012 Summer Olympics in London.
However, Federer certainly benefited from rival Rafael Nadal battling through injuries for a large part of the year, and some uncharacteristically poor play from Novak Djokovic at times.
Nadal defeated Federer in one Australian Open semifinal before losing to Djokovic in the final, then went on to win his seventh French Open title. However, the Spanish star didn't even play at the US Open because of injury.
When he returns next season fully healthy, Nadal will be the favorite in every match that he plays against Federer.
In addition to Nadal, Djokovic and Andy Murray are also playing at a high level. Murray is playing the best tennis of his life right now, and is a tougher opponent for Federer than he used to be. Murray has made the "Big Four" a true "Big Four."
Other talented players who are highly ranked, such as Jo-Wilfried Tsonga, David Ferrer and Berdych aren't easy opponents in Grand Slam tournaments.
The competition right now is deeper and better, and players such as Nadal and Djokovic are likely to have bounce-back seasons in 2013.
Federer could win another Grand Slam in 2013, it's not impossible by any stretch, but the challenge of winning one will likely be much harder than it was this season.