Paris: Novak Djokovic and Andy Murray can set up a Davis Cup quarter-final blockbuster if they successfully pilot Serbia and defending champions Great Britain through the first round.
However, the world's top two players will go into their home clashes against Kazakhstan and Japan respectively this weekend with question marks over their fitness and match sharpness.
World number one Djokovic saw his streak of 17 successive finals come to an end in Dubai last week when an eye infection forced a quarter-final retirement.
Murray, meanwhile, hasn't hit a ball in anger since losing to old rival Djokovic in the Australian Open final almost five weeks ago.
The 28-year-old took February off after his wife Kim gave birth to their first child, Sophia.
Djokovic led Serbia to the 2010 Davis Cup title.
"I am optimistic and I hope that I am going to play," said the 11-time major winner. "I was a little sceptical but now all is going well."
Djokovic and world number 23 Viktor Troicki will play singles for the Serbs in Belgrade while Kazakhstan's top player is Mikhail Kukushkin, the world 79.
Murray, who led Great Britain to their first Davis Cup title in 79 years last November, said being a new father has not affected his sleep patterns.
"It was very different to what I expected," he said. "I still practise hard and do all the same things I was doing before. I will find out if it is different on Friday or not."
The key clash in Birmingham could come on Sunday when Murray takes on Japan's top player, Kei Nishikori, the world number six.
Murray holds a 5-1 career advantage over Nishikori ahead of the first Davis Cup clash between the two nations since 1931.
"Britain have one of the best teams with Andy and his brother (doubles star, Jamie). It's a great challenge," said Nishikori.
- Hewitt plays it again -
Should Serbia and Britain emerge victorious, they will clash in the UK from July 15-17 for a place in the semi-finals with Murray already revealing that he intends to play.
In Melbourne, former world number one and two-time major winner Lleyton Hewitt could be drafted into action as a playing captain in the clash with the United States after Nick Kyrgios was ruled out through injury.
It's a match-up between the tournament's two most successful nations but the US won the last of their 32 titles in 2007. Australia captured their 28th and most recent crown in 2003.
In a shock move late Wednesday, Hewitt replaced Kyrgios in the squad despite having retired as a player after the Australian Open.
The 35-year-old is now ranked 301 in the world but boasts a formidable Davis Cup record of 42-14 in singles.
Bernard Tomic, the world 20, will play singles as will Sam Groth with Hewitt likely to only feature in Sunday's doubles on the grass courts of Kooyong.
Top 30 players John Isner and Jack Sock will lead the United States' hopes.
The winner of the tie will take on either last year's runners-up Belgium or 2005 champions Croatia who meet in Liege.
Switzerland, the champions in 2014, go to Pesaro to face Italy without Roger Federer, who is recovering from knee surgery, and the resting Stan Wawrinka.
Instead their hopes will be spearheaded by 34-year-old Marco Chiudinelli whose ranking is a lowly 146.
The winner of that tie will face either Poland or Argentina who are playing in Gdansk.
The Poles are in the World Group for the first time while Argentina, runners-up three times since 2006, are in the elite section for a 15th consecutive year.
The South Americans will be favourites to progress with two players -- Leonardo Mayer and Guido Pella both in the top 50. Poland's top singles player is Jerzy Janowicz ranked at 96.
Yannick Noah begins his second spell in charge of France in the Caribbean where Guadeloupe hosts the tie against Canada.
Nine-time champions France can count on top-10 stars Jo-Wilfried Tsonga and Richard Gasquet while Canada are reeling after world number 13 Milos Raonic withdrew with an adductor injury suffered at the Australian Open.
For France or Canada, a quarter-final with either Germany or the Czech Republic, the 2012 and 2013 champions, awaits.
They meet in Hanover with world number seven Tomas Berdych making his 30th Davis Cup appearance for the Czechs.