Aussie tennis star Nick Kyrgios has been forced to explain his decision to drop to a knee at the Laver Cup was not linked in any way to the protests swirling across the NFL.

The 22-year-old was spotted kneeling with his head bowed after the coin toss before his final match of the Laver Cup against Roger Federer on Monday morning (AEST) in Prague.

European media linked Kyrgios' kneel with the USA national anthem protests that have spread throughout the NFL.

However, after falling to Federer 4-6 7-6 (8-6) 11-9 in a blockbuster showdown, Kyrgios revealed his decision to take a knee had nothing to do with the protests.

Advertisement

NFL stars hit back on Monday (AEST) after President Donald Trump criticised players who kneel or sit during the US national anthem to protest civil rights abuses.

At games in London and around the US on Sunday, more than 100 players knelt, linked arms and gave black power salutes as The Star-Spangled Banner rang out.

The mass protests come after Trump urged fans to boycott NFL games to pressure the league to fire or suspend the "sons of bitches" who show disrespect for "flag and country."

Kyrgios said after his match that his kneeling to dedicate his performance to his grandmother and grandfather.

"I'm doing that before most matches just to remember, you know, the two most important people that have passed away," he said.

Kyrgios revealed in a column for The Player's Voice last week that he has previously made several tributes, including a hidden tattoo, for his grandmother Juliannah Foster, who passed away two years ago.

His grandfather Christos Kyrgios also passed away in April. It resulted in the World No. 20 pulling out of the Estoril Open and taking a short break away from tennis.

Kyrgios courted controversy in January when he wore a "F*** Donald Trump" T-Shirt while holding a press conference in Sydney.

It comes after Trump disinvited NBA champions Golden State Warriors from a visit to the White House after superstar Steph Curry indicated he did not want to attend a meeting with the President.

Trump's extraordinary responses to the NBA and NFL flashpoints have turned both situations into one giant crisis for his administration.

The protests in the NFL have seen former allies, including Super Bowl champion coach Rex Ryan and New England Patriots owner Robert Kraft, distance themselves from Trump.

The silent protests are spreading and are gaining momentum each time Trump publicly condemns them.

The Pittsburgh Steelers remained in the locker room as the national anthem played before their game with the Chicago Bears. Coach Mike Tomlin stood by himself on the sideline.

Tomlin had said before the game that Pittsburgh's players would remain in the locker room and that "we're not going to let divisive times or divisive individuals affect our agenda."

Tomlin added that the Steelers made this choice "not to be disrespectful to the anthem but to remove ourselves from this circumstance. People shouldn't have to choose."

Even New England Patriots owner Robert Kraft, who's been a strong supporter of the president, expressed "deep disappointment" with Mr Trump.

"Our players are intelligent, thoughtful, and care deeply about our community and I support their right to peacefully affect social change and raise awareness in a manner that they feel is most impactful," he said.

At least 20 Patriots players were spotted kneeling, as Tom Brady linked arms with his teammates.

The protests began as the NFL made their now annual pilgrimage to London.

About two dozen players, including Baltimore Ravens linebacker Terrell Suggs and Jacksonville Jaguars running back Leonard Fournette, took to a knee during the playing of the national anthem before the start of the game at Wembley Stadium.

No players were kneeling during the playing of the British national anthem, God Save the Queen.

Jaguars players stood arm-in-arm with team owner Shahid Khan, who donated $1.25 million to Mr Trump's inauguration fund.

Shortly after kick-off, Ravens owner Steve Bisciotti released a statement: "We recognise our players' influence. We respect their demonstration and support them 100 per cent. All voices need to be heard. That's democracy in its highest form."

TRUMP CALLS FOR BOYCOTT

President Trump took to Twitter to savage the NFL.

'FREE SPEECH ON OWN TIME'

US Treasury Secretary Steve Mnuchin defended Mr Trump's attacks on NFL players, saying members of the league "can do free speech on their own time."

"I think what the president is saying is that the owners should have a rule that players should have to stand in respect for the national anthem," Mr Mnuchin told This Week.

"This isn't about Democrats, it's not about Republicans, it's not about race, it's not about free speech. They can do free speech on their own time. That this is about respect for the military and first responders in the country."

REAL IMPACT OF TRUMP'S NFL WAR

US President Donald Trump's refusal to back down on his war protesting NFL and NBA stars has seen the situation fall further out of control.

Trump on Monday morning (AEST) stepped up his condemnation of the silent protests after insisting his concern with the protests has nothing to do with race relations in the United States.

Political commentators have declared Trump's aggressive response to the protests is certain to bolster the support for the athletes taking a stand.

However, the same commentators believe the spreading protests may not ultimately be harmful to his presidency if there is an eventual backlash against the spreading protests.

It is going to get a lot uglier before it gets better. The NFL stars aren't backing down either.

USA Today's Nancy Armour wrote Trump has bitten off more than he can chew, taking on two monster sporting organisations with a global audience.

"Trump's remarks Friday, and again Saturday and Sunday, were as ignorant as they were inflammatory, yet more racist dog whistles for his base," she wrote.

"Perhaps he counted on a league that has effectively blackballed Colin Kaepernick to let them pass in uncomfortable silence, but the NFL showed Sunday that he had badly overplayed his hand."

The Globe and Mail's Cathal Kelly writes Trump's response to the protests has radicalised NFL and NBA stars.

"Whatever equilibrium might have existed has been tipped over because Trump is Trump. His raison d'etre is provocation.

"All it took was a few rambling sentences and an early morning message on social media to radicalize a good portion of two major sports leagues.

"Mr. Trump's "son of a bitch" comment about kneelers and his disinviting of the NBA-champion Golden State Warriors have brought the NFL and NBA closer to open war with him. But that war has many fronts, and not all of them extend outward.

"By early Sunday afternoon, you could see a shift under way. The smattering of dissenters has become a large crowd, though it remains conspicuously racially divided."

'SONS OF BITCHES'

The fight began on Saturday (EST) at a Republican rally in Alabama when Mr Trump attacked activist NFL players - mostly African-Americans - as "sons of bitches" for kneeling or sitting during renditions of The Star-Spangled Banner.

On the same day, basketball star Stephen Curry, the top player for California's Golden State Warriors, said he would not attend a traditional White House reception honouring the winning basketball team.

Several hours later, President Trump hit back with an early-morning Twitter salvo.

"Going to the White House is considered a great honour for a championship team. Stephen Curry is hesitating, therefore invitation is withdrawn!" he wrote.

His petty outburst drew a stinging response from across the NBA, with Cleveland Cavaliers superstar LeBron James among the first to weigh in.

"U bum @StephenCurry30 already said he ain't going!" James wrote on Twitter.

"So therefore ain't no invite. Going to White House was a great honour until you showed up."

The protests began last year when quarterback Colin Kaepernick of the San Francisco 49ers refused to stand for the anthem to protest racial injustice and police brutality.

Several more players have since joined in, nearly all of them black. Kaepernick, who was unable to land a job with a team this season, has attracted support from his peers but also some backlash.

The 29-year-old said he started his protests because he wanted to spark a nationwide debate on racial injustice and police brutality.

Bruce Maxwell of the Oakland Athletics became the first MLB player to kneel during the US anthem just hours after Donald Trump's "fire them" comments.