Laver Cup: Jack Sock and Nick Kyrgios beat Rafael Nadal and Stefanos Tsitsipas 6-4, 3-6, 10-6.
Alexander Zverev came from a set down to beat Kevin Anderson as the Europeans successfully defended the title won in Prague last year.
Team World slashed Europe's overall lead to 7-5 heading into Sunday's third and final day of the 2018 Laver Cup.
The doubles dream team of Roger Federer and Novak Djokovic lost their match but Team Europe still led Team World 3-1 after the first day of Laver Cup.
Roger Federer has never lost a single match to any of the remaining players in the ATP World Tour Finals 2017.
Jack Sock is the first American to reach the semi-finals of the ATP World Tour Finals event since Andy Roddick in 2007.
Jack Sock, the American number one, came from nowhere to reach the ATP World Tour Finals, reaching London by winning the Paris Masters.
Roger Federer hasn't triumphed at the season-ending Tour Finals since 2011.
The 16th seed ended his Serbian opponent's fairytale run in the French capital to break into the world's top 10 for the first time and take the last place for London from Spaniard Pablo Carreno Busta.
American tennis player Jack Sock, ranked 18 in the world, took to Twitter to find the young fan from whom the towel was snatched.
John Isner entered the semi-finals of Paris Masters with a 7-6 (8/6), 4-6, 6-4 win over compatriot Jack Sock
Juan Monaco defeated Jack Sock in three sets to win the US Men's Clay Court Championships.
Jack Sock knocks out top-seed John Isner, to face Juan Monaco in final of ATP Houston open.
John Isner set up a quarter-final with compatriot Jack Sock after he entered the quarter-finals of the ATP Houston Open.
Jack Sock defeated Matthew Barton to advance to the quarter-finals of the ATP tournament in Texas.
Roberto Bautista Agut won the ATP Auckland Classic Title after Jack Sock was forced to withdraw early from the final due to flu.
Rafael Nadal hit 10 less winners than Jack Sock's 29 but was the more consistent of the two. He committed 10 less unforced errors than his opponent's 35.