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* China warns of counter measures against U.S. law
* Retail shares in focus as Black Friday kicks off
* U.S. stock markets to shut at 1 p.m. ET
* Indexes off: Dow 0.34%, S&P 0.27%, Nasdaq 0.25% (Changes comment, updates market action)
By Arjun Panchadar
Nov 29 (Reuters) - U.S. stocks slipped on Friday in a post-holiday shortened session as a U.S.-China discord over Hong Kong reignited trade tensions and retail stocks dipped on signs of a tepid start to in-store Black Friday sales.
China on Thursday threatened to retaliate against a U.S. law backing pro-democracy protesters in Hong Kong. The measures could include barring drafters of the legislation from mainland China, Hong Kong and Macau, the editor of China's state-backed Global Times tabloid said in a tweet.
The trade-sensitive Philadelphia Semiconductor index fell as much as 0.8% and was on track for its worst day in a week.
The diplomatic clash also knocked Wall Street's main indexes off record highs. They had closed at all-time highs in every session so far this week on upbeat domestic data and hopes of an imminent "phase one" trade deal.
"The fact that this is a half day with the markets being at all-time highs is reason to give investors a bit of a pause," said Robert Pavlik, chief investment strategist, at SlateStone Wealth LLC in New York.
Sentiment was also dulled by reports of spot checks on retailers around the country, which showed fewer people lining up outside stores at the start of Black Friday, suggesting a surge in online buying may have taken the shine off America's biggest shopping day.
The S&P 500 retail sector fell 0.7%, with Kohl's Corp dropping 2.3%. But top retailers Walmart Inc and Best Buy Co Inc were up between 0.3% and 0.7%.
At 11:48 a.m. ET the Dow Jones Industrial Average was down 95.45 points, or 0.34%, at 28,068.55, while the S&P 500 was down 8.63 points, or 0.27%, at 3,145. The Nasdaq Composite was down 21.73 points, or 0.25%, at 8,683.45.
Shares of Tech Data Corp jumped 12.3% as private equity firm Apollo Global Management raised its bid for the U.S. information technology equipment distributor to about $5.14 billion.
Declining issues outnumbered advancers for a 1.58-to-1 ratio on the NYSE and for a 1.18-to-1 ratio on the Nasdaq. The S&P index recorded 15 new 52-week highs and one new low, while the Nasdaq recorded 66 new highs and 26 new lows. (Reporting by Arjun Panchadar and Uday Sampath in Bengaluru; Editing by Arun Koyyur)