Are you kidding? Some of the greatest players ever played in his era.
Who did Federer have to deal with, especially from 2003-2007? An almost retired and aged Agassi and an up coming Nadal were his opponents. Since the competition became tougher, Federer barely succeed like he did prior. Just compare him to Nadal since 2008-2014:
Nadal 10 GS
Federer 5 GS
Sampras' H2H:-
vs krajicek :4-6
vs stich : 4-5
vs bruguera : 2-3
vs hewitt : 4-5
vs safin : 4-5
vs roddick : 1-2
Let's look at his rivals,
Agassi --- A good case for him being a chief rival even though he wasn't mentally there during Sampras' prime (meth, daddy issues etc.)...Teenage Nadal did more in 2005-2007 than Agassi from 1986-June 1999. Talk about having a worthy main rival.
Already dominating clay (3 French Open titles and barely any losses)
Already reaching 2 Wimbledon finals (as many as Agassi did in his entire career)
Getting better and better on hard courts with a couple of MS titles and Slam QF appearances
Bruguera --- two majors on
clay in 93' and 94'. Irrelevant as Sampras was a non-factor on clay.
Courier --- Two majors on
clay in 91' and 92' (Irrelevant as Sampras was a non-factor on clay.) & Two AO's in 92' and 93' (The two AO wins qualify him to be one of Sampras' rivals)
Goran --- Won his only Wim in 2001 when Sampras was wayy past his prime. Roddick is crucified for losing to Federer in important matches after winning a major and being number 1 but Goran is a greater rival than Roddick, Hewitt, Safin even though he was never able to overcome Agassi and Sampras in their primes and was never ranked # 1?
Becker --- won his last major in 1996. Even though he won his last major in 96', he did so by not beating Sampras who lost to Philippoussis in the fourth round...If Federer had lost in the fourth round at the 2006 USO, Roddick could've won his second major. Becker's prime was late 80's,90' and 91'. his prime did not coincide with Sampras' prime.
Edberg -- won his last major in 1992. Only one major coincides with Sampras' prime or his era of dominance. Can you really call him one of his rival? I don't think his prime coincided with Sampras'
Rafter --- Winner of USO in 97' and 98'. Good argument for Rafter to be a rival.
Muster --- Won his only major on
clay in 1995. Irrelevant as Sampras was a non-factor on clay.
Chang --- Won his only only major on
clay in 1989. Irrelevant as Sampras was a non-factor on clay and this win was way before Sampras won his first major.
So really I would only consider Agassi, Courier and Rafter to be Sampras' rivals. Even though there were other big names...They were just that...Big names who were past their prime and/or won a surface where Sampras never had a chance to begin with.