Seventy-eight players have now qualified for the 156-man field for the June 13-16 event at Merion Golf Club outside Philadelphia ahead of final sectional qualifying on June 3.
Day, the 2011 US Open runner-up to Rory McIlroy, and Weekley were among 25 players to join the field by being within the top 60 in world golf rankings.
Weekley vaulted 56 positions in the world rankings to No. 55 by winning the US Tour's Colonial Invitational on Sunday, while Day was safely in with his ranking at 25.
Manassero became exempt for the US Open with his victory in Sunday's four-hole play-off at the European Tour PGA Championship.
Others qualifying through the rankings included Tim Clark of South Africa, Belgian Nicolas Colsaerts, Scotland's Martin Laird and Paul Lawrie, Ian Poulter, Australian Marc Leishman, Swede Henrik Stenson and Thongchai Jaidee of Thailand.