Hillary Clinton versus Donald Trump is expected to be one of the most-watched events in US political history.

Experts say viewership for the first in a series of televised debates could be close to 80 million people.

But with the eyes of America, and the world, on the stage, what do both controversial candidates need to do to win the duel?

Here are the key areas both Clinton and Trump need to target:

Hillary Clinton

Appeal to undecided voters

A recent poll by the Wall Street Journal and NBC News showed that around 20% of voters were undecided or leaning towards a third, alternative candidate.

Clinton will have to convince the stray voters to side with her or at least persuade them to steer clear of Trump.

Inspire loyal Democrats

Clinton has to come out strong on the issues that are vital to across the party’s core base.

She is expected to outline her commitment to social justice issues and as well as reforms to the immigration system.

Point the finger at Trump

Clinton, who is no stranger to controversy, is the second least popular major party candidate in the history of modern polling – fortunately for her, Trump is the first.

It’s important that Clinton stays on Trump and doesn’t allow him to redefine himself as a candidate that appeals to swing demographics, such as suburban women.

She will have to throw his long list of previous provocative statements in his face and also challenge him on claims he makes that are false during the debate.

Donald Trump

Be unpredictable

One of Trump’s biggest assets during his campaign is his unpredictability – he must use that to trip up his rival.

He has already started, in fact. Over the weekend, he sent a very public invite on Twitter to Gennifer Flowers, with whom Bill Clinton reportedly had an affair with decades ago, asking her to be his guest at the debate.

Be presidential

He has hit the headlines for a variety of reasons over the past 15 months and it’s served him well to reach this point.

But now, Trump must back up his bluster and bravado with hard policies and knowledge which shows he can be trusted to head up one of the most powerful nations on Earth.

Point the finger at Clinton

In much the same way, Mr Trump must take advantage of his opponent’s unpopularity.

Mrs Clinton has been hit by a number of crises throughout her candidacy, including the email scandal and questions over the Clinton Foundation fundraising practices.

But it’s important the Republican candidate sticks to the fact and doesn’t make outlandish claims which undermine the real issues facing Mrs Clinton.