Pete Sampras stopped caring as much about tennis, after he won his 13th Slam at Wimbledon in 2000. There he broke Emerson's record, and was pretty much ready to check out of tennis and call it a career.
The next couple years you could really see a decline in Pete's game. He was a step slower, and he didn't have the intensity he used to have.
Even when you watch Pete's last match where he won the 2002 US Open against Agassi, you can see that was not the same Pete Sampras we were used to seeing in his Prime.
(But Pete still made the Finals of the US Open 3 years in a row: 2000, 2001, 2002)
Infact he was already ready to let guys like Hewitt & Safin take over the sport. Pete was happy to retire.
Btw, if the US Open didn't have that stupid back-to-back Saturday(semi) & Sunday(final) deal, Sampras may have actually won it in 2000(vs Safin) and 2001(vs Hewitt). And Sampras may have ended up with 16 Slams. Btw, Sampras did beat Safin the following year. And along the way to the 2002 US Open Final, Sampras beat Roddick, and Agassi beat Hewitt. Which shows how even in their old age, Sampras and Agassi were still able to beat the young top players.(Hewitt was ranked #1, Safin was ranked #2. Agassi was ranked #3 in the world at that time. Sampras was ranked outside the top 10)
Anyway, my point is that this illustrates that transitional period in tennis where Sampras was clearly not the same dominant player he used to be and ultimately retired. Which allowed Agassi, even in his advanced age, to take advantage of that, and win 2 more Slams in 2001(AO) and 2003(AO) and he made the US Open Final versus Federer in 2005. A post-prime Agassi reaching the US Open Finals, and challenging a prime Federer to 4 sets, once again signifies how great of a player Agassi used to be in his Prime.
In other words, the fact that Agassi was still playing successfully past his prime, tells us that Hewitt, Safin, and Roddick would be no challenge for a Prime Agassi, Sampras, Rafter, etc.
Therefore, the years of 2000-2007 was a relatively weak era in tennis. And one man benefitted greatly... Roger Federer. No one was a true test for him during his years of dominance. There was no true talent playing in that time span who was comparable to a Prime Agassi or a Prime Sampras or a Prime Rafter. And it made Federer look like a God out there beating up against 3rd rate players. And everyone got fooled into thinking Federer is the GOAT.
Then in 2008, the Greatest Match in History took place at Wimbledon and changed everything.
Enter Rafa Nadal. Finally someone who could challenge Federer and beat him, and beat him, and beat him.
Someone on the level of a Prime Sampras, a Prime Agassi, a Prime Rafter.
Enter Novak Djokovic. Once Nole entered his Prime, he finally was able to challenge and beat Federer as well.
We can add Andy Murray to the mix as well, with his winning record over Fed and the beatdown he gave him at the Olympics.
Winning wasn't easy anymore for Roger Federer.
No more Hewitts, Safins, Roddicks to beat up on now.
I respect the 5 Grand Slams that Federer won from 2008 on.
But I don't respect the 12 cheap Grand Slam wins he racked up from 2003-2007.
A Prime Sampras, Agassi, Nadal, and Djokovic could've done the same thing in that weak field era.