TOKYO, March 24 (Reuters) - Japan's Nikkei share average was flat at the morning session's close on Monday as a modest rise in Wall Street's three main indexes last week marginally boosted market sentiment, but a looming deadline for implementing additional U.S. tariffs kept investors cautious.

The Nikkei was unchanged from Friday's close of 37,676.97 by the midday break, while the broader Topix fell 0.3% to 2,794.67 after logging an eight-month peak at the end of last week.

The Nikkei received some support as a portion of heavyweight shares saw some buying. Uniqlo parent firm Fast Retailing ( FRCOF ) rose 0.4% and AI-focused startup investor SoftBank Group climbed 2.9%.

However, the benchmark index struggled to advance, seesawing between gains and losses as market participants eyed the April 2 deadline that U.S. President Donald Trump has set for reciprocal and sector-specific tariffs to go into effect.

Trump hinted that he would be flexible regarding the new round of levies, helping lift the S&P and Dow at the end of the week, but Japanese equity investors on Monday took a wary approach to the impending threat.

Many important details regarding the latest tariffs are still unknown, keeping investors on edge, said Kazuo Kamitani, a strategist at Nomura Securities. "There's no way to predict it, so we have no choice but to wait for the announcement," he said.

Many major semiconductor-related shares dragged on Monday to follow their U.S. peers after the Philadelphia SE Semiconductor Index ended nearly 1% down on Friday.

Tokyo Electron ( TOELF ) slid 0.3%, while Screen Holdings ( DINRF ) stumbled 3.3% to become the biggest percentage loser on the Nikkei.

Among other big-name shares, Seven & i Holdings ( SVNDF ) rose 2% and Chugai Pharmaceutical ( CHGCF ) climbed 1.6%. Robot maker Fanuc ( FANUF ) gave up 1.7%.

Of the Nikkei's 225 constituents, 66 advanced while157declined. Two shares were untraded.

(Reporting by Brigid Riley; Editing by Janane Venkatraman)