Andre Agassi may possess eight grand slam titles but the American is still seeking to lift his game to a higher level.
Despite having more major titles then any other active rival, the 33-year-old is nevertheless driven by his ambition to be a more complete player.
"For me, it's never been about winning tournaments. It's always been about trying to get better as a tennis player," said Agassi, who became the first overseas player to win four Australian Open crowns in January.
"I think it's important for every athlete, when their career is done, to be able to say they got the most out of their game, that they got to their best level, whether it's for a week or whether it's for five years.
"I want to always push myself to get better and to feel like I'm playing tennis I've never played before. That's my motivation."
But for the oldest man to top the rankings since they were introduced in 1973, chasing that dream does not get any easier.
"It's very important for me to choose my tournaments wisely," he said. "I don't have energy to burn anymore.
"It's important for me to feel fresh and ready to go."
With mental and physical fatigue becoming a major factor in his 17th year as a professional, Agassi can understand why his great rival Pete Sampras announced his retirement on Monday.
"It's easy for me to identify with what has motivated Pete in the past, and what's not motivating him now," said Agassi.
"It's not easy to do this, to travel the world all year round when you've done it so many times.
"You compromise everything in your life on a daily level, with all the weeks of training, the weeks of preparation.
"Sometimes your family pays the price for what you have to do to be at your best. There's a big price tag that's involved. Certainly the longer you do it, the harder it gets."