That would be the 2008 tour to Sri Lanka 2nd test at Galle. It's arguably his greatest test innings, have no idea how does he play spin like that, a batsman from my college cricket team told me once that you cannot play a turning ball unless you are technically sound and patient. Sehwag has his ways of dealing with things though.Was there another Test that series where he absolutely destroyed Ajantha Mendis?
2nd Test: Sri Lanka v India at Galle, Jul 31-Aug 3, 2008 | Cricket Scorecard | ESPN Cricinfo