BGod
G.O.A.T.
For this exercise I'm only looking at semifinal matches at Slams that went 4 or 5 sets, so I'm not hypothetically considering reversals of straight set victories as they were definitive. I also didn't look at quarters as that's asking 2 things to go differently and I tried to spread out time periods and players as hypothetical Nadal and Federer matchups were done in another thread.
2011 US Open Nadal vs. Federer
Federer had a rather underrated 2011 season. He only lost to Djokovic at AO & USO, Nadal at the French and a 2-0 choke against Tsonga at Wimbledon. He would go on to win the WTF. Of course his 2011 US Open is the concentration here. Nadal tore the tour open in 2010 and despite Federer having double match point in that semifinal against Novak, it was on Novak's serve and Federer was playing worse. His 2011 form was on fire, dropping a single set to Cilic he went up 2-0 and held double match point on his serve. We all know what happened and Novak went on to beat Nadal rather easily. Obviously the fact Nadal and Federer have yet to meet at the US Open elevates this hypothetical matchup to stratospheric heights indeed.
2006 Australian Nalbandian vs. Federer
Okay, maybe this is a stretch but remember Nalbandian was one of the few who actually matched up well against Federer, beating him at the 03 USO and in close proximity to this would be clash at the 2005 WTF spoiling Fed's quest to best McEnroe's mark. David was up 2-0 against Baghdatis and lost by a break in the 4th and 5th sets. It really is too bad as this match-up at worst would have had juicier story-lines but could have seen an upset as Fed had dropped sets to Davydenko, Kiefer and needed 5 sets against Tommy Haas. Certainly could have been an all-time affair.
1995 US Open Becker vs. Sampras
Like Fed/Nadal, Becker and Sampras never met at the US Open despite two close brushes in 1990 and 1995, both times Becker losing to Agassi in 4. It's hard to choose between the two, Becker seemingly with an advantage in 95 and Sampras with an advantage in 95 however I look at Becker's resurgence in 95-96 and think the match would have been good. It was in 96 the two had 5 set wars on carpet winning one each and Becker had that run in Australia to start the year. He also played Andre much closer in 95 than 90 and I'm sure wanted revenge for the Wimbledon Final. Boris was in good form and arguably had a tougher road, beating Andre would have meant he was at least up to a fight.
1993 Wimbledon Edberg vs. Sampras
If you thought somebody could get into Sampras' head it was Edberg. They only met in 2 Slam matches and Edberg won both, at the US Open Final in 92 and the 93 Australian. Their total head to head was 8-6 in favour of Sampras but the last 3 meetings which Pete won have to be taken with a grain of salt and still Edberg pushed in two tight 3 set affairs (Bo3). Otherwise Edberg lead 5-3 at the onset and clearly won their two Slam meetings. He was two years removed from a champion on grass but lost a tight 5 setter to Ivanisevic in 92 (who went on to beat Sampras) and a VERY tight 4 setter to Stich in 91 who went on to win it all (6-4, 6-7, 6-7, 6-7). He had the mental edge against Sampras at this point and Pistol Pete had not yet tasted Wimbledon glory.
1991 Australian Becker vs. Edberg
Yes, yes, they played eachother 35 times but rarely at Slams and never at either Flushing Meadows or Melbourne. In the Aussie, Becker went on an interesting run including a 5 set thriller against Camporese in the 3rd culminating in him dismantling Lendl in the Final. Edberg on the other hand beat pre-ascension Courier in 5 sets in the 4th Round and had the lead against Lendl before losing the last 2 sets. Edberg had won their only French meeting in 5 sets in 89 and had the 2-1 Wimbledons but in Australia in 1991 I think the edge would have been with Becker. It may have served as the decider with Becker having a 25-10 head to head but 1-3 in Slams. Regardless it would have been a better show than Becker beating Lendl again.
1990 US Open McEnroe vs. Agassi
Old lion against young lion. These legends only met 4 times, splitting the series albeit Mac's 89 WCT win came via retirement after Andre won the opening set. Their only Slam match was the 1992 Wimbledon semi which Agassi won with ease on route to his maiden title. Obviously it's a bit of a pipe dream to imagine Mac beating Sampras in hindsight but he did only lose the 2nd set 4-6 before winning the 3rd so it's not an impossibility. Agassi had lost back to back semifinals to Lendl and was somewhat nervous in final. Even if this was a foregone conclusion it could have changed Agassi's career dramatically but I suspect given their styles this may have been a great match.
2011 US Open Nadal vs. Federer
Federer had a rather underrated 2011 season. He only lost to Djokovic at AO & USO, Nadal at the French and a 2-0 choke against Tsonga at Wimbledon. He would go on to win the WTF. Of course his 2011 US Open is the concentration here. Nadal tore the tour open in 2010 and despite Federer having double match point in that semifinal against Novak, it was on Novak's serve and Federer was playing worse. His 2011 form was on fire, dropping a single set to Cilic he went up 2-0 and held double match point on his serve. We all know what happened and Novak went on to beat Nadal rather easily. Obviously the fact Nadal and Federer have yet to meet at the US Open elevates this hypothetical matchup to stratospheric heights indeed.
2006 Australian Nalbandian vs. Federer
Okay, maybe this is a stretch but remember Nalbandian was one of the few who actually matched up well against Federer, beating him at the 03 USO and in close proximity to this would be clash at the 2005 WTF spoiling Fed's quest to best McEnroe's mark. David was up 2-0 against Baghdatis and lost by a break in the 4th and 5th sets. It really is too bad as this match-up at worst would have had juicier story-lines but could have seen an upset as Fed had dropped sets to Davydenko, Kiefer and needed 5 sets against Tommy Haas. Certainly could have been an all-time affair.
1995 US Open Becker vs. Sampras
Like Fed/Nadal, Becker and Sampras never met at the US Open despite two close brushes in 1990 and 1995, both times Becker losing to Agassi in 4. It's hard to choose between the two, Becker seemingly with an advantage in 95 and Sampras with an advantage in 95 however I look at Becker's resurgence in 95-96 and think the match would have been good. It was in 96 the two had 5 set wars on carpet winning one each and Becker had that run in Australia to start the year. He also played Andre much closer in 95 than 90 and I'm sure wanted revenge for the Wimbledon Final. Boris was in good form and arguably had a tougher road, beating Andre would have meant he was at least up to a fight.
1993 Wimbledon Edberg vs. Sampras
If you thought somebody could get into Sampras' head it was Edberg. They only met in 2 Slam matches and Edberg won both, at the US Open Final in 92 and the 93 Australian. Their total head to head was 8-6 in favour of Sampras but the last 3 meetings which Pete won have to be taken with a grain of salt and still Edberg pushed in two tight 3 set affairs (Bo3). Otherwise Edberg lead 5-3 at the onset and clearly won their two Slam meetings. He was two years removed from a champion on grass but lost a tight 5 setter to Ivanisevic in 92 (who went on to beat Sampras) and a VERY tight 4 setter to Stich in 91 who went on to win it all (6-4, 6-7, 6-7, 6-7). He had the mental edge against Sampras at this point and Pistol Pete had not yet tasted Wimbledon glory.
1991 Australian Becker vs. Edberg
Yes, yes, they played eachother 35 times but rarely at Slams and never at either Flushing Meadows or Melbourne. In the Aussie, Becker went on an interesting run including a 5 set thriller against Camporese in the 3rd culminating in him dismantling Lendl in the Final. Edberg on the other hand beat pre-ascension Courier in 5 sets in the 4th Round and had the lead against Lendl before losing the last 2 sets. Edberg had won their only French meeting in 5 sets in 89 and had the 2-1 Wimbledons but in Australia in 1991 I think the edge would have been with Becker. It may have served as the decider with Becker having a 25-10 head to head but 1-3 in Slams. Regardless it would have been a better show than Becker beating Lendl again.
1990 US Open McEnroe vs. Agassi
Old lion against young lion. These legends only met 4 times, splitting the series albeit Mac's 89 WCT win came via retirement after Andre won the opening set. Their only Slam match was the 1992 Wimbledon semi which Agassi won with ease on route to his maiden title. Obviously it's a bit of a pipe dream to imagine Mac beating Sampras in hindsight but he did only lose the 2nd set 4-6 before winning the 3rd so it's not an impossibility. Agassi had lost back to back semifinals to Lendl and was somewhat nervous in final. Even if this was a foregone conclusion it could have changed Agassi's career dramatically but I suspect given their styles this may have been a great match.