At the 2018 US Open, the semifinals had three previous winners on the men's side.
Has it ever happened in a major tournament that all four places were held by previous winners?
If not, has it happened in any ATP tournament?
Some other that came close :
2015 US Open. 3 were previous winners and 1 was a future (next year) winner
2012 Australian Open. 3 were previous winners and the 4th was Murray who lost 2 finals before 2012 and 3 after
2011 US Open. 2 were previous winners, Djokovic won it that year and Murray the year after
2009 US Open was a unique one, in the SF were : 2008(Federer), 2009(Del Potro), 2010(Nadal) and 2011(Djokovic) champions
2008 US Open had the same guys from 2011 (Big 4)