* Tesco, Sainsbury sales up 3.9 pct in 6 wks to Jan. 6
* UK grocery market up 3.8 pct over period
* Asda sales up 2.1 pct, Morrisons down 1.0 pct
LONDON, Jan 15 (Reuters) - Britain's biggest retailer Tesco and No. 3 grocer J Sainsbury dead-heated in the battle for sales growth in the key Christmas period, according to data published on Tuesday.
Market researcher Kantar Worldpanel said Tesco and Sainsbury both posted identical gross sales growth rates of 3.9 percent in the six weeks ending Jan. 6, with market shares unchanged from a year ago.
It said the overall grocery market grew 3.8 percent, driven by price inflation rather than volume growth.
"This represents a clear improvement in Tesco's fortunes following a series of share dips in 2012 but it is also a strong performance from Sainsbury's which had been predicted to suffer disproportionately from the Tesco fight-back by some commentators," said Kantar Worldpanel director Edward Garner.
Last week Kantar published data for the 12 weeks to Dec. 23 which showed Sainsbury's posting the highest sales growth of the so called "big four" grocers, which also include No. 2 player Wal-Mart's Asda and No. 4 Wm Morrison.
Also last week, Sainsbury and Tesco both published their own Christmas trading statements and argued over who had performed the best over the festive period.
Kantar said Asda, which is not due to report until February, saw sales growth of 2.1 percent over the six week period, while Morrisons' sales fell 1.0 percent.