An ominous Rafael Nadal was in cruise control for Spain at the ATP Cup on Monday but Serbia's Novak Djokovic was forced to sweat through brutal conditions for his second win in a row. The Spanish world No 1 swept past Uruguay's experienced Pablo Cuevas 6-2, 6-1 in just 73 minutes in Perth to ensure his country won their second consecutive tie at the inaugural 24-team event and moved closer to the quarter-finals. He came on court after teammate and world No 10 Roberto Bautista Agut made short work of Franco Roncadelli 6-1, 6-2. "A great match. I played better than the other day [when he beat Nikoloz Basilashvili 6-3, 7-5]. It was a very positive victory against an opponent for who I have a lot of respect," said 19-time Grand Slam winner Nadal. "I'm super happy." Spain now take on Japan on Wednesday with the winner qualifying for the knockout phase in Sydney, while the loser will have to rely on other results. Djokovic was given a testing workout in his opening clash in Brisbane on Saturday by big-serving South African Kevin Anderson, who took him to two tie-breakers. He was back to his dominant best against France's Gael Monfils, but the sapping heat and humidity made the 6-3, 6-2 win harder than the score suggested. "It was definitely one of the most humid conditions that I have ever had in my career. Tonight was brutal," said the world No 2, who extended his record over Monfils to 16-0, dating back 15 years. "Obviously, playing Gael, you know you are going to have a lot of exchanges, a lot of rallies. He is a great fighter and a great friend of mine. Tough luck today." Djokovic then teamed up with Viktor Troicki to defeat Nicolas Mahut and Edouard Roger-Vasselin 6-3, 6-7, 10-3 in the deciding doubles match to send Serbia into the quarter-finals. Benoit Paire had put France on the board earlier by overcoming Dusan Lajovic 6-2, 6-7, 6-4. Eight of the 24 nations will make the quarter-finals in Sydney from the round robin – six group winners and two best runners up. Other winners on Monday included Japan, Croatia, South Africa and Austria. World No 4 Dominic Thiem bounced back from a shock loss in his opening match to beat tenacious Diego Schwartzman in two tough sets to steer Austria to victory over Argentina in Sydney. Earlier, Dennis Novak showed battling qualities to upset world No 25 Guido Pella after losing the first set 6-0. It kept Austria alive in Group E after they lost their opening tie to Croatia. The unbeaten Croats are sitting pretty, spearheaded by former US Open champion Marin Cilic who eased past Poland's Kacper Zuk in straight sets – his second victory in a row. "I felt that the level was quite good. I was being aggressive," said Cilic, a losing Australian Open finalist in 2018. "I had to stay mentally in the match, really focused." One of the biggest surprises has been Japan, which has won both its ties – against Uruguay and Georgia – despite being without injured stars Kei Nishikori and Yasutaka Uchiyama, their third best player. Yoshihito Nishioka has stepped up, beating world No 26 Nikoloz Basilashvili in straight sets in Perth after thrashing world number 45 Pablo Cuevas on Saturday. "I think I played my best tennis and very happy about this," he said of his win against the Georgian where his pinpoint accuracy rattled Basilashvili. South Africa, spearheaded by Anderson, remain in contention after clean-sweeping Chile 3-0 in Brisbane. Former world No 5 Anderson clinched the tie by beating Cristian Garin 6-0, 6-3 after Lloyd Harris's comfortable win over Nicolas Jarry. Raven Klaasen and Ruan Roelofse made it a whitewash when they battled past Garin and Jarry in the doubles 1-6, 6-3, 10-7. "I think it's been promising. The things I'm doing, the things we're working on seeing," said Anderson, who missed the second half of last year after suffering a knee injury at Wimbledon. "We have made a few changes with a few things and so far so good."