Reddy completes his maiden Test fifty as he lofts Starc over the off-side for a couple of runs, from 81 balls.
He again hits the next ball uppishly past mid-off for two more runs.
In the next over, Sundar slaps Cummins through the off-side for three runs to raise the 50-run stand for the eighth wicket.
A single each to Reddy and Sundar as India avoid the follow-on.
The last ball is full and swings into Reddy, who is struck on the pads, but the confident shout for leg before is turned down.
Australia have one review but they decide against the DRS as the ball could have gone over the stumps.