Nick Kyrgios' girlfriend Costeen Hatzi says he's 'still a winner in my eyes' after his second-round loss to world No. 2 Daniil Medvedev

Russian tennis star Daniil Medvedev beat Nick Kyrgios in a four-set win at the Australian Open on Thursday.

And following his second-round loss, Nick's girlfriend Costeen Hatzi shared a tribute to her beau on Instagram, saying he put on a winning performance despite losing to the world No. 2.

'So proud of you. Played an amazing match... Still a winner in my eyes,' the 21-year-old wrote.

Nick went Instagram official with Costeen last month, sharing a photo on his grid of the couple cuddled up together.

Meanwhile, Medvedev blasted the Australian Open crowd for booing him during his four-set win against Aussie favourite Kyrgios.

The Russian came through 7-6, 6-4, 4-6, 6-2 to reach the third round but had a dig at the spectators immediately after the match concluded, particularly for booing in between his first and second serve.

Yet during the first week in Melbourne, many onlookers have actually been screaming Cristiano Ronaldo's iconic 'Siuuu' celebration, not booing, but Medvedev was not convinced.

'I came here to win this match and I'm happy I managed to do it... that's the only choice when you get booed between first and second serve, it's not easy so you just had to stay calm and win the match,' he said to former Grand Slam winner Jim Courier on court.

After Courier tried to explain to the Russian that the crowd could have been doing Ronaldo's famous 'Siuuu' celebration - which has been common so far at Melbourne Park - Medvedev was unmoved in his view of the crowd.

'Sorry, I can't hear you. Guys, I can't hear him, show some respect for Jim Courier. He won here, guys, thank you.

'Let him speak please, guys, if you respect someone at least respect Jim Courier.

'Between first and second serve it's not easy. I remember it was the case on the games I lost and the break points, it's just tough to play.'

Later, speaking to Eurosport, the outspoken Medvedev elaborated on the crowd: 'It's not anger, it's a little bit disappointing. I suppose it's normal especially when you play a home favourite and not just the home favourite but Nick.

'I had a break point, second serve, and people are cheering like I've already made a double fault. And that's just disappointing because it's not everybody who is doing it, but those who are doing it probably have a low IQ.'