Vincent justified his selection with unbeaten 86 at WACA

Debutant opener Lou Vincent justified his selection with an unbeaten 86 on day one of the deciding third cricket Test against Australia at the WACA today.

New Zealand put on 103 for the session to go to tea at 2-190, with Vincent unbeaten on 86 and captain Stephen Fleming on 79.

The pair came together in the first hour of play with the Kiwis in trouble at 2-19 and Australian quicks Glenn McGrath (1-15) and Jason Gillespie (1-36) looking dangerous.

But the tide swung in the visitors' favour after nine overs when McGrath limped from the field and was taken to hospital for precautionary scans on his back.

Vincent, who went to lunch level with Fleming on 33 not out, lifted his scoring rate after lunch. Fleming beat him to 50, but then the plucky right hander became more aggressive after he was bounced by frustrated leg-spinner Shane Warne (0-40).

He signalled his intention to punish the Australian attack the very next ball when he hooked a Brett Lee (0-71) bouncer for six.

He is closing in on his maiden century, but a century on debut has never been a particularly good omen for Kiwi openers.

Jack Mills, who scored 117 in his first Test against England at Wellington in 1929-30, went on to play just six more.

Second on the all-time list was left-handed Rodney Redmond, who smashed 107 on debut against Pakistan at Auckland in 1973, his only Test.

Australia's spirits received a boost 43 minutes into the second session when paceman Glenn McGrath returned to the field after a lengthy absence.

McGrath will be assessed again during tea, but is unable to bowl until 4.10pm (7.10pm AEDT).