Let's analyze the 2010 Nadal's season with empirical facts:
Roland Garros: strong competition. He defeated 25-year-old Soderling in the final, the only man in the history of humankind who has beated peak Nadal at Roland Garros. Soderling also had defeated Federer at RG 2010, so he was in form.
Soderling was diagnosed with Mononucelosis in August 2011, not in 2010.
https://www.theguardian.com/sport/2015/dec/24/robin-soderling-retires-tennis-glandular-fever.
Wimbledon: weak but in form opponent in the final (Berdych). Berdych had destroyed both Federer and Djokovic at Wimbledon 2010. If Federer and Djokovic couldn't defeat Berdych, let alone Nadal. Nadal dominated Federer and Djokovic those days on grass.
US Open: strong competition. Nadal faced Djokovic in the final and defeated him. Now, some people put the excuse that Djokovic was not peak at the US Open 2010. I disagree. He started his peak the last months of 2010, he even beated Federer at the SF of the US Open. It's not like he was non-peak the 31th of December of 2010 and suddenly became a peak player the 1st of January of 2011.