1. 1976: Orantes d. Fibak 5–7 6–2 0–6 7–6 6–1, after losing in group stage
2. 1978: McEnroe d. Ashe 6–7 6–3 7–5, winning both matches
3. 1981: Lendl d. Gerulaitis 6–7 2–6 7–6 6–2 6–4, winning both matches
4. 1989: Edberg d. Becker 4–6 7–6 6–3 6–1, after losing in group stage
5. 1990: Agassi d. Edberg 5–7 7–6 7–5 6–2, after losing in group stage
6. 1994: Sampras d. Becker 4–6 6–3 7–5 6–4, after losing in group stage
7. 1996: Sampras d. Becker 3–6 7–6 7–6 6–7 6–4, after losing in group stage
8. 1999: Pete Sampras d. Agassi 6–1 7–5 6–4, after losing in group stage
9. 2000: Kuerten d. Agassi 6–4 6–4 6–4, after losing in group stage
10. 2001: Hewitt d. Grosjean 6–3 6–3 6–4, winning both matches
11. 2003: Federer d. Agassi 6–3 6–0 6–4, winning both matches
12. 2004: Federer d. Hewitt 6–3, 6–2, winning both matches
13. 2005: Nalbandian d. Roger Federer 6–7 6–7 6–2 6–1 7–6, after losing in group stage
14. 2008: Djokovic d. Davydenko 6–1 7–5, winning both matches
15. 2011: Federer d. Tsonga 6–3 6–7 6–3, winning both matches
16. 2015: Djokovic d. Federer 6–3 6–4, after losing in group stage
17. 2017: Dimitrov d. Goffin 7–5 4–6 6–3, winning both matches
18. 2018: Zverev d. Djokovic 6–4 6–3, after losing in group stage

So 2020's final between Thiem and Nadal is the 19th rematch in the tournament's history. Historically, the group stage loser has won the final 10 out of 18 times, although in the 21st century it's been 7-3 in favour of the group stage winner. It's also been 4-2 for the group stage winner when the final was a Bo3 match (1978, 2008 on).