New Zealand batsman Kane Williamson kept his head while all around him were losing theirs, reaching an unbeaten century to steer New Zealand to a tense five-wicket win over Sri Lanka in the second test on Monday in Hamilton.

New Zealand secured a clean sweep of the two-match Test series.

Williamson ended 108 not out after resuming at 78 as New Zealand progressed from 142-5 overnight to reach its winning target of 189 after only 55 minute play on the fourth morning of a match which saw dramatic changes of fortune.

Both teams seemed to squander winning positions before New Zealand finally edged to brink of victory by the end of the third day Sunday on which 16 wickets fell.

BJ Watling remained not out on 13. Williamson and Watling did not allow any wickets to fall in the morning and safely guided their team to victory.