Questions about world No. 1s Novak Djokovic and Serena Williams, as well as the absence of old dependables Roger Federer and Maria Sharapova, have made the 2016 US Open not quite as closed an event as it has seemed in recent years.

A year ago Djokovic entered Flushing Meadows having reached all three Grand Slam finals, only losing at Roland Garros to Stan Wawrinka. Williams had won all three 2015 major titles and appeared on the verge of a calendar Grand Slam.

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This year there are concerns about Djokovic’s left wrist and Williams’ right shoulder, ailments that forced both of them out of the Western & Southern Open in Cincinnati last week. Both injuries appear to have pre-dated the Olympics where Djokovic lost in the opening round to Juan Martin del Potro and Williams went out in the third round to Elina Svitolina. The Serb also made a surprising third-round exit to Sam Querrey at Wimbledon but appeared to bounce back by winning Rogers Cup in Toronto on July 31.

Williams turns 35 on September 26 and Djokovic will only play two more Grand Slams – this year’s US Open and next January’s Aussie Open – before becoming a 30-year-old. By Roland Garros next year he will be 30 and male players accumulating large numbers of Grand Slams over 30 is not common. Andre Agassi’s 30-and-over wins at the 2001 and 2003 Aussie Opens are something of an exception.

Federer has played 19 Grand Slams since turning 30 in August 2011, winning only once – at Wimbledon in 2012.

Williams has been a shining star in terms of not slowing down with age. She has won nine of her 22 Grand Slam titles after her 30th birthday.

Champion in 2015 at the US Open with a win over Federer in the final, Djokovic was surprisingly upset in the semifinals the previous year by Kei Nishikori on a hot, humid afternoon – definitely not Nole’s preferred conditions.

As for Williams, last year there was incredible pressure on her to complete the Grand Slam – during the Flushing Meadows fortnight ESPN memorably had “Serena” and not “tennis” or “US Open” on its bottom screen crawl of various sports. In hindsight it all became a little too much for Williams in her semifinal against spunky, enterprising Roberta Vinci.

With the 2016 US Open beginning next Monday, she and Djokovic remain the favourites with the bookmakers. Here are odds with the British-based Ladbrokes firm.