The science behind Federer and Serena's never-ending careers

World number one Serena Williams, at age 33 and 10 months, became the oldest woman in the modern era to win a singles' Grand Slam at this year's Wimbledon.

Roger Federer, also 33, would've become the oldest men's champion in the Open Era had he triumphed against Novak Djokovic in the final.Still going strong at their age and giving much younger opponents a run for their money, one can't help but admire the longevity of these two great champions.

"They're ageing beautifully like vintage wine, both of them," remarked one commentator during the Wimbledon coverage. It's true that they're both exceptional athletes and extremely gifted tennis players but their prolonged success can actually be explained in less subjective terms. In conversations with some sports physicians during my time at Boston University (BU) I learnt that, as athletes age, their type II muscle fibres - called fast-twitch, associated with strength, power and speed - reduce significantly more than type I muscle fibres, which contribute to physical endurance.

It means that while older players lose a step or two once they cross a certain age, their endurance levels are still pretty high and, if you're a tennis player, that's good news. Just ask Martina Hingis. Persistent injuries forced her to retire in 2002 at the age of 22, a time when most players are breaking through. But she came back this year to win the Wimbledon women's doubles and the mixed doubles titles aged 34.

The way modern tennis is being played with long, punishing rallies, slower courts and marathon matches over the last decade or so, it has effectively become an endurance sport. This is especially the case at the Grand Slam stage, where the men play best-of-five-sets instead of the regular three. To put numbers on that, Rafael Nadal and Djokovic engaged in a brutal 54-shot rally at the 2013 US Open final, which was unheard of in the serve-and-volley days of Boris Becker and Stefan Edberg. The two also contested the longest Grand Slam final in history, clocking 5 hours and 53 minutes at the 2012 Australian Open.

Long gone are the days when a teenager could win Wimbledon, like Boris Becker did from 1985-86 and Maria Sharapova in 2004. The statistics seem to concur. According to a study released in the Journal of Quantitative Analysis in Sports last year, there was a four-fold increase in the percentage of top-100 male tennis players aged 30 or above over the last 20 years. Men's tennis has also seen an increase in the average age of top singles players from 25 years in the 1990s to 28 years now.

The study noted that "the age of tennis' highest-ranked singles players is now comparable to the age of elite long-distance runners and this evolution likely reflects changes in tennis play that have made endurance and fitness increasingly essential for winning success."

Sport physicians suggest that endurance athletes such as the modern day tennis players also typically peak in their late 20s or early 30s which is why the likes of Spain's David Ferrer and Italy's Francesca Schiavone posted their best career results close to the 30 age mark.