Serena Williams headlines a heavyweight field at the Rogers Cup as the WTA's best seek to build momentum ahead of the Olympic Games.

The Premier 5 event at Stade Uniprix in Montreal represents the final high-profile tournament before the battle for coveted medals in Rio next month. 

Wimbledon champion Williams will be the favourite to claim a fourth Rogers Cup title and first in Montreal, her previous three in 2001, 2011 and 2013 all having come in Toronto, which hosts the competition in alternate years.

Williams suffered a semi-final upset at the hands of surprise winner Belinda Bencic last year but the Swiss teenager will not be able to defend her title due to a wrist injury.

"I am very sad to have to withdraw from the Rogers Cup," she said in a statement last week. 

"I was looking forward to coming to Montreal and defending my Rogers Cup title but unfortunately my wrist has not completely healed. I will miss my Canadian fans and I am looking forward to getting back on the court soon."

Another high-profile absentee is pregnant world number six Victoria Azarenka, who announced last week she is taking a break from tennis as she awaits the birth of her first child.

An elbow injury means Caroline Wozniacki, the 2010 winner, is also out.

World number five Simona Halep will be aiming to make up for the disappointment of losing to Bencic in the 2015 decider.

Unlike many of her fellow WTA heavyweights, the Romanian will not be using the Rogers Cup as preparation for the Olympics - the 24-year-old has withdrawn from the Rio Games due to fears over the Zika virus.

And that leaves this event as a major priority for the Wimbledon quarter-finalist ahead of the US Open.

Angelique Kerber, Garbine Muguruza and Roberta Vinci join Halep as the major threats in the bottom half of the draw.

It remains to be seen if Wimbledon finalist Kerber can mount a meaningful challenge as she battles an elbow problem that forced her to withdraw from the Swedish Open.

World number one Williams and sister Venus are in the top half of the draw, together with Dominika Cibulkova, Carla Suarez Navarro and Agnieszka Radwanska. Promising young American Madison Keys could also prove to be an obstacle barring the path of either Williams sister to the final. 

Canada's number one Eugenie Bouchard, ranked 41st in the world, opens her campaign on home soil against Lucie Safarova.