Half-centuries from VVS Laxman and Mahendra Singh Dhoni helped India build a substantial first-innings lead on the third day of the second Test against South Africa at the Eden Gardens.
Laxman hit a classy, unbeaten 97 and Dhoni was batting 15 runs short of a century as India exerted control on a match they must win to draw the series.
At the tea interval, India had extended the lead over South Africa's first-innings total of 296 to 252, reaching 548 with four wickets still intact.
South Africa suffered a frustrating start in the morning when nightwatchman Amit Mishra and Laxman - the overnight pair - denied them early success and steadily built up a 48-run partnership.
Mishra started confidently, stepping out to cart Harris for a boundary over long-on and slashing Jacques Kallis for two more in one over.
India's cause was aided by some sloppy work in the field by South Africa, however, as Mishra was grassed by Kallis on five and again by Graeme Smith at cover on 25.
The tourists eventually took the new ball and Steyn looked threatening, but it was Morne Morkel who struck as Kallis held an excellent catch at second slip to remove Mishra.
Dhoni then joined Laxman and although South Africa's pacemen bowled well, they were let down by more lapses in the field.
Dhoni began cautiously and edged Harris while on 23, but the ball flew safely between wicketkeeper AB de Villiers and Kallis at slip.
Dhoni amassed runs quickly after the lunch interval, needing only 77 deliveries to reach his half-century.
Laxman, who had inside-edged Morkel just past a diving De Villiers, was dropped again by JP Duminy on 48 and he went on to reach his seventh consecutive half-century.
The runs continued to flow easily in the afternoon session as the duo ground South Africa's attack into the dust, so far amassing 164 for the seventh wicket as India went through another session in this Test without the loss of a wicket.