TOKYO, Sept 30 (Reuters) - Japanese shares fell on Wednesday, tracking decline in U.S. stock futures after a combative debate between President Donald Trump and Democrat Joe Biden increased concerns about an indecisive election outcome.

The Nikkei 225 Index dropped 1.5% to 23,185.12, pulling back from a seven-month high reached in the previous session. The broader Topix ended down 1.97% at 1,625.49.

Both candidates battled fiercely over Trump's leadership on the coronavirus pandemic, the economy and the integrity of the upcoming election.

Neither candidate emerged from the debate with a clear advantage in what is expected to be a close election that may not yield an apparent winner, which is unsettling investors in riskier assets such as equities, traders and analysts said.

"Uncertainty about the U.S. election has actually increased," said Daiju Aoki, regional chief investment officer for Japan at UBS Securities.

"Trump had been doing better in the polls but Biden was able to make his points and appeal directly to voters. Increased uncertainty should weigh on Japanese equities."

The underperformers among the Topix 30 were Mitsubishi UFJ Financial Group Inc down 4.59%, followed by Mizuho Financial Group Inc losing 3.78%.

The stocks that gained the most among the top 30 core Topix names were wireless carrier NTT Docomo Inc up 20.92%, followed by venture fund SoftBank Group Corp gaining 0.5%.

NTT Docomo shares surged for a second day after its parent NTT launched a $40 billion bid to take the wireless carrier private. There were 19 advancers in the Nikkei index against 205 decliners.

Sentiment is likely to remain weak because the presidential election is extremely hard to predict, meaning it is difficult for investors who are already long in equities to increase their positions further, traders said.

The volume of shares traded on the Tokyo Stock Exchange's main board was 1.44 billion, compared to the average of 1.14 billion in the past 30 days. (Reporting by Stanley White; Editing by Rashmi Aich and Sherry Jacob-Phillips)