Miami - Roger Federer's comeback from injury has been so successful that his compatriot Stan Wawrinka believes he can become the oldest world number one in history at the age of 35.
Federer beat Wawrinka in the final of Indian Wells on Sunday for his second win of the year following his triumph in the Australian Open - his 18th Grand Slam title and his first in four and a half years.
Those wins have propelled the Swiss master to sixth in the world as he makes excellent progress after a six-month layoff caused by a knee injury.
Compatriot Wawrinka believes that not only is Federer back in form but he is playing in a noticeably different manner that gives him a chance to return to the pinnacle of the rankings.
"He's playing really well, what is different maybe is he is playing closer from the baseline, preferring, using less his slice, he is using more top spin, putting more pressure all the time, returning better that's the difference I seem," Wawrinka said on Tuesday at the Miami Open.
"For sure he has a shot for number one, he has just won the first Masters final. For sure he has a big shot," he told reporters.
Andre Agassi holds the record for being the oldest player to hold the men's No 1 ranking, at the age of 33.
"It is going to be interesting to see. Rafa (Nadal) is playing well also and we will see when Novak (Djokovic) and Andy (Murray) are back from injury, it is going to be really interesting for tennis fans," Wawrinka said.
Wawrinka said it wasn't a shock to see Federer, who now has 90 career titles, back to such form but that he was particularly impressed by his movement.
"From him nothing is a surprise, but for sure to see him moving that well at that age it is something amazing. It is good for me to see that you can be at the top (at that age). It is amazing to see how he is playing after a six-month injury."
Federer said he was well ahead of his target of reaching the top eight before Wimbledon and said he was enjoying the very different feeling of success late in his career.
"At 25 I was winning 95% of my matches and eventually you are just on this train where you just keep rolling and you just expect yourself to win a lot, you play many more tournaments and you are just racing from one to the next, it was an incredible experience to be able to keep that high level for so long and be beating so many other guys and win so many finals in a row, I had some unbelievable records," he said.
"It is definitely very different, this year is very different to any other year that I have had, last year was also very different with the injury. But with the age, the comeback, I think this is very very special for me and I definitely see things differently to how I ever have (in the past), it is normal when you realise that you are not going to have another 15 years to play on tour," he said.
Federer could face a strong challenge from old rival Nadal who is seeking his first ever win in Miami after losing in four finals at the Key Biscayne venue.
The Swiss, a winner in 2005 and 2006, is the only former winner in the field and is looking for a third sweep of Indian Wells and Miami titles.