Rival coach takes aim at Kyrgios player's box

The coach of Nick Kyrgios' opponent Grigor Dimitrov has questioned whether the Aussie was getting illegal coaching from his box on Sunday night.

Daniel Vallverdu could be seen gesturing towards the chair umpire during the second set of the blockbuster fourth round clash at Melbourne Park, clearly upset by something.

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Dimitrov was already one set up at the time, with the Aussie looking to hit back in the second.

Kyrgios could be heard talking to his team throughout the match but there was no suggestion he was receiving illegal advice back from them.

Lleyton Hewitt suggested in commentary that it could have been a deliberate tactic from the Bulgarian's team to unsettle the often volatile Aussie.

"I feel Dani may have been asking though if Nick got a code violation for coaching that something like that might set Nick off," Hewitt said.

Kyrgios wasn't warned over the incident but coincidentally he was broken in the next service game, before going down in another tie-breaker as Dimitrov jumped to a commanding two sets to love lead.

The match began brightly, with Dimitrov holding his first three service games of the match comfortably as Kyrgios fought off two break points in the sixth game to tie it up.

Both players were on song with their serves as the first set flew towards an inevitable tie-breaker.

The third-seeded Bulgarian grabbed a crucial mini break early on the tie-breaker and never relinquished his advantage as he saw it out 7-3 to claim the opening set.

The 22-year-old Aussie didn't let the disappointment derail him as he began the second set strongly by jumping out to a 2-1 lead.

Kyrgios had Dimitrov down 30-0 on his own serve but the Bulgarian answered the challenge in style to put the pressure back on the Aussie.

The second set continued in a similar vein to the opening stanza with neither man willing an inch on their own serves.

A repeated foot fault infringement threatened to hammer the nail in Nick Kyrgios' coffin in the third round when the Aussie was pinged with his second foot fault of the night.

"I haven't been called for a foot fault for the last three years," an irritated Kyrgios shot back at the chair umpire.

"That's twice today. It's not possible man."

Replays showed that the Aussie's trailing right foot did arguably stray across the centre of the service square, with commentator Jim Courier conceding he thought the officials got the call right.

In further evidence of the 22-year-old's improved maturity, Kyrgios brushed aside the frustration to hold his serve before breaking Dimitrov as he hung on in the match.

The Aussie's serve continued to cause his rival problems and the 22-year-old made the most of the break to claim the third set 6-4 - keeping his tournament hopes alive.

Dimitrov had a chance to serve for the match at 5-3 up in the fourth set but Kyrgios broke before taking his next three service games to love - forcing yet another tie-breaker.

However, the Aussie's joy was short-lived as Dimitrov again upped the ante to take the breaker 7-4 and with it, the match.

The Bulgarian next faces Britain's Kyle Edmund in the quarter-finals.