Novak Djokovic put Serbia on course for the Davis Cup semi-finals Friday before admitting that seeing his career plunge into freefall since his history-making French Open triumph had been "hard to accept".
The world number two shrugged off his recent elbow injury -- and his mediocre record of just seven wins in 2017 -- to give 2010 Davis Cup champions Serbia a winning start in their quarter-final against Spain in Belgrade with a 6-3, 6-4, 6-2 victory over 24th-ranked Albert Ramos Vinolas.
Viktor Troicki, the world 39, then made it 2-0 by seeing off 19th-ranked Pablo Carreno Busta 6-3, 6-4, 6-3.
Victory for Djokovic was a major confidence boost for the former world number one who has lost his Wimbledon, US and Australian Open titles in the last year and surrendered the top ranking spot to Andy Murray.
"I have lost my best game in the last few months," Djokovic, whose French Open win in June last year allowed him to complete the career Grand Slam, told reporters after securing a 12th successive Davis Cup singles win.
"It was hard for me to accept after being on top for the last six years but today's match shows that I can get back to winning ways.
"I am very happy with my performance today which is very encouraging because I now know I am on the right path to discovering my best form."
Five-time Davis Cup winners Spain are without Nadal in Belgrade after the 14-time Grand Slam champion opted to stay at home to prepare for the clay-court season.
Team captain Nenad Zimonjic and Troicki will face Marc Lopez and Jaume Munar in Saturday's doubles where a win will give Serbia a semi-final date against either France or Britain.
On a perfect day for all four of the quarter-final hosts, Australia were 2-0 up on the United States in Brisbane, closing in on a semi-final date against Belgium who were 2-0 ahead of Italy in Charleroi.
- France, Australia, Belgium in charge -
In Rouen, Lucas Pouille and Jeremy Chardy moved nine-time champions France to the verge of a fifth semi-final in eight years with straight sets wins against a British team missing Andy Murray who sat out the tie with an elbow injury.
Pouille claimed a 7-5, 7-6 (8/6), 6-3 victory over Kyle Edmund before Chardy eased past Dan Evans, playing on clay for the first time in three years, 6-2, 6-3, 6-3.
Chardy, only France's ninth best singles player, was competing in his first Davis Cup singles rubber in six years.
France can wrap up the tie on Saturday when Nicolas Mahut and Julien Benneteau face Jamie Murray and Dominic Inglot in the doubles.
"I am always nervous, but I am a lot more relaxed now than I was yesterday," said France skipper Yannick Noah, who has won the Davis Cup both as a player and captain.
Nick Kyrgios and Jordan Thompson gave Australia an unexpected 2-0 lead against the United States in the countries' 47th meeting in the competition.
World number 79 Thompson stunned world number 15 Jack Sock 6-3, 3-6, 7-6 (7/4), 6-4 before Kyrgios saw off John Isner 7-5, 7-6 (7/5), 7-6 (7/5).
The controversial Kyrgios pushed Roger Federer to the brink in the Miami semi-finals last week and he carried that form into Friday's match.
"I'm trying to knuckle down and compete for every point and that's something I haven't done in the past," said Kyrgios.
"I'm still very emotional out there. I'll still challenge the umpire, I'll still say what I think, I'll still throw my racquet, I'll still swear, but I'll compete every point."
In Charleroi, Steve Darcis beat Paolo Lorenzi 6-7 (3/7), 6-1, 6-1, 7-6 (7/4) while David Goffin eased past Andreas Seppi 6-4, 6-3, 6-3 as Belgium led Italy, who knocked out defending champions Argentina in the first round.