SYDNEY-- Daniil Medvedev continued to baffle his opponents at the ATP Cup on Thursday, securing a hard-fought and fiery victory against Argentina's Diego Schwartzman and propelling Russia into the semi-final of the inaugural team event.
Medvedev, ranked five in the world and undefeated in singles this tournament, won 6-4 4-6 6-3 in Sydney and will lead Russia against the winner of Novak Djokovic-led Serbia or Canada.
Earlier, Russia's Karen Khachanov defeated an out-of-sorts Guido Pella from Argentina 6-2 7-6(4), with the victories giving Russia an unassailable 2-0 lead in the quarter-final tie ahead of the doubles match.
The decisive victory contrasted with the knife-edge contest in the day's other quarter-final between Australia and Britain.
Australia's untested doubles pairing of Alex De Minaur and Nick Kyrgios saved four match points against Britain's Jamie Murray and Joe Salisbury before clinching a spot in the semi-finals in a half hour-long extended tiebreak - used in place of a deciding third set.
With Russian team captain Marat Safin sitting in his corner, Medvedev broke his opponent's serve twice - and gave one break back - in an entertaining first set, where his slapping forehand ebbed between sublime and out-of-control.
World number 13 Schwartzman got better as the match progressed, attacking relentlessly.
The pair exchanged heated words at one point, earning Team Russia a warning, before Medvedev received a point penalty after waving his racquet at the umpire and hitting the umpire's chair shortly after losing a service game in the second set.
Medvedev said after the match he had been frustrated with how his opponent reacted to a stroke of luck during the match, which prompted his exchange with Schwartzman and, later, the umpire.
Medvedev said he deserved the reprimand from the umpire and that while he expected to be fined by the tour, he did not anticipate further disciplinary action.
"Nothing happened to the chair, nothing happened to anybody, I didn't break my racquet," he said.
The crowd turned against Medvedev after the incident, and the Russian looked ready to implode, though it proved to be only a partial flare-up.
Schwartzman won the second set but Medvedev recovered to take the decider, with the 1.98m (6-feet-6-inch) Russian increasingly relying on his booming serve to get himself out of trouble when tested.
In the day session, De Minaur and Kyrgios, better known for their singles play, defeated doubles specialists Murray and Salisbury 3-6 6-3 18-16 to set up a semi-final against either Rafael Nadal-led Spain or David Goffin's Belgium.
In something resembling both a hug and a wrestle, De Minaur and Kyrgios fell to the ground in a celebratory embrace after edging the half hour-long extended tiebreak, used in place of a deciding third set.
"That was the most stressful tiebreaker I've ever been involved in," Kyrgios said in an on-court interview after the match. "I'll definitely be having a couple of red wines tonight."
The pair were subbed in by team captain Lleyton Hewitt at the last moment after the teams split the two singles matches, a decision he attributed to Kyrgios' irresistible form.
While Kyrgios' on-court behaviour often raised hackles, few handle pressure moments as well as he did on Thursday. Facing a match point in the doubles tiebreak, he ripped a backhand winner off a serve down a narrow gap.
In the singles matches, Kyrgios was too strong for London-based left-hander Cameron Norrie, winning 6-2 6-2, while Britain's Dan Evans then outlasted the top 20-ranked De Minaur 7-6(4) 4-6 7-6(2).
Hewitt then made the prescient call to put the 20-year-old De Minaur immediately back out on court, partnered with Kyrgios, instead of doubles specialists.