Nick Kyrgios will pose a serious threat to the United States' Davis Cup aspirations in their quarter-final tie with Australia, Jim Courier has warned.
US captain Courier believes it will take "something special" for his team to overcome Kyrgios, who has been in excellent form since his second-round exit from the Australian Open in January.
The world number 16 has beaten Novak Djokovic twice this year in reaching the quarter-finals of the BNP Paribas Open and last four of the Miami Open, with both of those runs ended by a resurgent Roger Federer.
And Courier knows the enigmatic 21-year-old will take some stopping when the last-eight tie begins in Brisbane on Friday.
"Nick has been playing some of the best tennis on tour this season so he's in great form and he should have a lot of confidence," Courier said. "We're going to have to do something special to beat him."
Courier's team - which includes the likes of Jack Sock, John Isner Sam Querrey - thrashed Switzerland 5-0 in the opening round and will be fancied to overcome opponents whose singles line-up is completed by world number 79 Jordan Thompson.
But Courier said there would be no danger of taking anything for granted, with Thompson having won both his singles matches in the 4-1 triumph over the Czech Republic in February.
"We've lost to a lot of players ranked lower than someone like Jordan Thompson so there won't be any complacency on our end," Courier said.
"We've seen too many surprises; we've beaten Roger Federer and Stan Wawrinka in this competition while we were ranked below and lost to players outside the top 100, so everything's up for grabs in Davis Cup."
The Americans prevailed in a first-round tie between the two nations in 2016