Nick Kyrgios is back in action at Wimbledon on Monday, closing in on a mouthwatering semi-final against Rafael Nadal as tournament chiefs said they had appealed a hefty fine imposed for banning Russian and Belarusian players.

Maverick Australian Kyrgios toppled Stefanos Tsitispas in a heated four-set thriller on Saturday, during which he called for the fourth seed to be kicked out of the event for hitting a ball into the crowd.

Kyrgios called the umpire a "disgrace" as tempers frayed and the bad feeling spilled over into the post-match press conferences.

The defeated Greek player said his opponent has an "evil side" and described him as a "bully", comments that Kyrgios laughed off.

Kyrgios, 40th in the world, thrives on his bad-boy image and Saturday's outbursts were not even his first of the tournament.

But it will be a different challenge against American Brandon Nakashima, a player he has never faced before, and he may struggle to re-create the big-match intensity on Centre Court.

Kyrgios, who has only reached two Grand Slam quarter-finals, believes he has the firepower to win Wimbledon.

"Round by round, if I keep doing my things, I feel good. I'm all right," said the 27-year-old, who had vocal support from the crowd on Saturday despite his antics.

Nadal, chasing the third leg of a calendar Grand Slam, found his rhythm in his third-round match against Italy's Lorenzo Sonego and now faces Dutch 21st seed Botic van de Zandschulp.

The 22-time Grand Slam champion, twice a winner at the All England Club, swept into the last 16 in straight sets after needing four sets to progress in his opening two matches.

"My best match, without a doubt, since the tournament started," said Nadal, the only top-10 player left in his side of the draw.

In the other two last-16 matches, Australia's Alex De Minaur plays Cristian Garin of Chile while Australia's Jason Kubler takes on 11th-seeded American Taylor Fritz.

Russian appeal

Wimbledon chief executive Sally Bolton, speaking before play started on Monday, said tournament bosses had lodged an appeal against fines handed down by the WTA women's tour for bans on Russian and Belarusian players. 

The All England Club imposed the sanctions in response to Russia's invasion of Ukraine and players also prevented from playing in Wimbledon warm-up events.

Both the ATP men's tour and the WTA reacted to the Wimbledon ban by stripping the event of ranking points.

The Daily Mail reported that the Lawn Tennis Association, which runs the sport in Britain, was fined $750,000 and the All England Club $250,000.

"It's subject to a legal process so I can't comment specifically but what I will say is that we stand by the decision we made," said Bolton. "We have appealed."

Quarters beckon for Tan 

Kazakhstan's Elena Rybakina reached the quarter-finals for the first time with a 7-5, 6-3 win over Petra Martic of Croatia.

She will face either Alize Cornet of France or Australia's Ajla Tomljanovic for a place in the semi-finals.

Harmony Tan, who defeated Serena Williams in the first round, takes on American Amanda Anisimova.

Tan, ranked 115th, needed more than three hours to see off Williams but on Saturday she hit top form, demolishing Britain's Katie Boulter 6-1, 6-1 in just 51 minutes.

The most eye-catching match of the day in the top half of the draw is a contest between fourth seed Paula Badosa of Spain and 2019 champion Simona Halep, who is yet to drop a set.

Romania's Halep, who is the 16th seed this year, said becoming a champion at the All England Club gave her confidence.

Badosa is the only member of the world's top 10 -- along with Tunisia's Ons Jabeur -- left in the women's tournament.