Once you reach the quarters or semis, the competition between now and then is very close. What makes the competition tougher TODAY IMO is that the competition is stiffer from the very first round onward. Every round is a potential danger zone. Not as true back then. Players today have to be on point every single round. No freebies.
I'm not sure why so many bring up this incorrect assumption. Maybe you didn't follow the game closely in the 90s. As someone who gambled on tennis quite a bit then (& now) the odds that top players would lose early in an event were much closer then compared to now(taking Fed out of the equation since almost every match he plays has the odds of Tyson-Douglas)
I'll do some research & start a new post. But from day 1 in 1990 there was great depth from 1-100. There were no freebies in any round, in any event. I don't even know where to start. The #1 & #2 seeds lost in the 1st round of the 1990 French Open. Andres Gomez won the '90 French. The #12 seed won the 1990 US Open. #1 Edberg lost in the 1st round of the 1990 US Open. There were so many unseeded players in the semis/finals of majors in the 90s. Guys like Wally Masur, David Wheaton, Henrik Dreekman, Magnus Larsson, Cedric Pioline, Jason Stoltenberg, Mark Woodforde, Todd Woodbridge, Jonas Bjorkman, Filip Dewulf, Patrick McEnroe, Wayne Ferreira, Alberto Berasategui, Alexander Volkov, Karel Novacek, Marc Rosset, Karol Kucera, Felix Mantilla, Jonas Svensson. 1999 French Open Final-Agassi(14) def Medvedev(100). 1996 Wimbledon Final- Krajicek(14) def Washington(19). 1997 French Open Final- #66 Kuerten beats #19 Bruguera. 1997 US Open-2 players outside the top 10(#14 Rafter & #20 Rusedski) played the final! Even if Fed weren't around, I can't imagine an all darkhouse final like that happening today, can you? And you do realize that top players are more protected today with the 32 seed format in slams?
Guys like Carretero & Woodruff won Masters Series events in the 90s.
There are many legit reasons to think Federer is a better player than Sampras. Tougher matches from round 1 on is not one of them. If he had it so easy, why did he struggle in so many early round matches? Heck, why did he lose to Yzaga, Korda, & Krajicek in majors if there wasn't great depth?