Ding, who won last week's Haining Open, led 4-2 and 5-4 but could not stop Day clinching the last two frames.
World number one Mark Selby beat Anthony McGill 6-1, and there were also wins for Shaun Murphy and John Higgins.
Scotland's Graeme Dott won the battle of former world champions as he beat Englishman Peter Ebdon 6-3.
Australia's Neil Robertson saw off Fergal O'Brien 6-3, last year's runner-up Mark Allen of Northern Ireland beat England's Kyren Wilson 6-3, but former world champion Mark Williams of Wales was edged out 6-5 by Thailand's Thepchaiya Un-Nooh.
See the full results and schedule for the 2015 International Championship
Welshman Ryan Day beat home favourite Ding Junhui in a final-frame decider to reach round three at the International Championship in Daqing, China.