Yoshihito Nishioka notched his second upset to start the season after downing world number 26 Nikoloz Basilashvili in straight sets Monday, as injury-hit Japan continued to surprise at the ATP Cup.
The world number 72 won 6-2, 6-3 in Perth to clinch the tie for unbeaten Japan after Go Soeda prevailed over Georgia's Aleksandre Metreveli 4-6, 6-3, 6-2.
Japan, who whitewashed Uruguay on Saturday, surprisingly top Group B in the 24-nation event despite the withdrawal of injured star Kei Nishikori and Yasutaka Uchiyama, their third best player, before the tournament.
The top eight countries from the round robin go onto a knockout phase in Sydney.
Nishioka, 24, has relished the extra responsibility, having beaten Basilashvili and world number 45 Pablo Cuevas, who he thrashed 6-0, 6-1 on Saturday.
"I think I played my best tennis and very happy about this," he said of his win against the Georgian.
The left-handed Nishioka's pinpoint accuracy rattled Basilashvili, who constantly misfired in an error-strewn first set before settling.
Basilashvili broke in the third game of the second set but was unable to consolidate, as Nishioka overcame service struggles of his own to prevail in straight sets.
In the earlier match, veteran Soeda was made to work by Metreveli, who rebounded after a horror debut at the ATP level when he failed to win a game against Spanish world number nine Roberto Bautista Agut on Saturday.
Metreveli, ranked 678, was broken early before finally getting on the board in the fourth game to ignite his confidence. He then broke back before dictating with aggressive groundstrokes to claim his first set on tour.
Both players struggled to hold serve in a see-saw start to the second set, but Soeda levelled the match after taking advantage of Metreveli's susceptible forehand and cruised from there.
"I was really tight in the first set, but I tried to focus after that," Soeda said. "There was so much pressure to win for the team, so I was nervous."
Soeda replaced Nishikori, who pulled out of the tournament and the Australian Open with an elbow problem.
A Rafael Nadal-led Spain play Uruguay in the later Group B tie.
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