I need to think of earlier decades but here goes the last 4 decades: 1970s: Borg v. Connors Wimbledon final 1977. This marked the turning point in the rivalry of the 2 outstanding players of the 70s, and also from this point on Borg probably took over from Connors as the best player in the world (whatever the computer says: Borg was the best player in the world from Wimbledon 77-Wimbledon 81). And the match was so close, it could have easily gone the other way. If Connors had won this Wimbledon the legacies of Borg and Connors looks different with Connors on 3 Wimbledons, and Borg on 4. But I wonder if Borg might have lost confidence playing at Wimbledon. I think he could get very discouraged by defeats. I don't think the Ashe 75 victory over Connors was as significant. It didn't mark a turn-around in the Ashe-Connors rivalry, being a 1 off upset. And though Ashe may be considered the true no.1 of 75 on the basis of his Wimbledon victory, I still think Connors played the highest quality of tennis that year. I don't include Riggs v. King because it was an exhibition, though very important. 1981 McEnroe v. Borg Wimbledon final and 1985 Lendl v. McEnroe US Open final Both matches not only decided who was the year end no.1 but also marked the beginning of lengthy reigns as no.1 and a drastic turnaround in rivalries. 1990s 1995 US Open final Sampras v. Agassi This match stopped Agassi's momentum in its tracks just as it looked as if he was about to topple Sampras. Agassi took over as no.1 in 99 due to Sampras' injury. 2000s so far 2006 Federer v. Nadal Wimbledon final Absolutely essential win to prove that he's the true no.1. I don't think the 2001 Federer v. Sampras match is because I think if Federer hadn't won Sampras would still not have won Wimbledon. With the tennis he was playing, Sampras wouldn't have got past Tim, Goran and Rafter.