Some eagle-eyed fans believe that tennis power couple Alex de Minaur and Katie Boulter are now engaged after the British star was seen wearing a ring on her ring finger.

Alex de Minaur is out of the Australian Open after letting his chance to reach the quarter-finals for the first time slip in a dramatic five-set capitulation to world No.5 Andrey Rublev.

The local hope fought back from losing the first set to snag a two-sets-to-one lead at Rod Laver Arena on Sunday and appeared destined to kick on as a frustrated and emotional Rublev waged a verbal war on himself.

But a brilliant Rublev came home with a wet sail to level proceedings then, as de Minaur crumbled, put the foot down in the fifth set to win 6-4 6-7 (5-7) 6-7 (4-7) 6-3 6-0.

The Aussie star's girlfriend Boulter was in the player's box throughout the match, and seemed just as nervous as every Australian tennis fan watching at home.

Cameras showed her reactions throughout the match, and some viewers spotted a large ring on her ring finger.

'KATIE BOULTER AND ALEX DE MINUAR ENGAGED !!! HE PUT A RING ON IT LOOK!' one fan declared confidently,' posted one fan on social media.

'Is Katie Boulter wearing an engagement ring?' replied another.

'KATIE BOULTER AND ALEX DE MINUAR ENGAGED!!! BLING BLING RING RING! CONGRATS,' wrote a third.

Earlier this month, questions over when De Minaur is going to take his relationship with Boulter to the next level stopped the Aussie star in his tracks.

Comedian Ash Williams, affiliated with 'The ATP Podcast,' put the big question to de Minaur at a press conference.

He asked: 'Will we get a proposal to Katie after the AO [Australian Open]?'

'Jeez. Putting me on the spot here,' de Minaur replied.

 'Yeah, it's not on the cards just yet. Not ruling it out. For the time being, I'm going to try to focus on playing tennis and then we'll think about the off-court activities.'

Williams persisted, asking: 'If you win the AO?'

'It wouldn't be a bad way to celebrate, let's just say that,' replied de Minaur.

De Minaur and Boulter met on the tournament circuit, initially keeping things casual until a coffee date three years ago changed everything.

Boulter last week said that all the attention the glamour couple are receiving in Australia takes some getting used to.

'It's definitely been fun to be a part of because it's been a little bit of a circus, but a good circus,' said Boulter.

'I can't walk down the street with him separately. I feel like I'm sharing him with everyone, which isn't a bad thing.

'I think it's awesome that he's getting the attention that he really does deserve. He's put the work in for it. He's playing some of the best tennis of his life.'