Nick Kyrgios and Spain's Fernando Verdasco traded insults on social media after the Australian said he wanted his Tour rival to lose at the Miami Open.
Kyrgios called Verdasco "the saltiest dude" as the Spaniard argued with Thanasi Kokkinakis during his last-32 win over Kyrgios' compatriot.
Verdasco responded by saying Kyrgios 'lacked courage' for later deleting his tweet, which also mocked Verdasco's record against Australian opponents.
The pair could meet in the semi-finals.
Verdasco, ranked 39th in the world, argued with Kokkinakis in the final set of his 3-6 6-4 7-6 (7-4) win after accusing the Australian qualifier's father of talking during his serves.
The often-outspoken Kyrgios, 22, then tweeted: "I hope TK wins this match, Verdasco is the saltiest dude, must be frustrated at his past success against Aussies."
Verdasco, 34, had lost his previous six matches against Australian players, including his two meetings with Kyrgios in 2015 and 2016, before beating Kokkinakis.
After the near three-hour match, Verdasco replied: "When you have the courage to put a tweet insulting another player you need to have the same to don't delete it."
That sparked another response from Kyrgios, who said: "I would honestly have told it to Fernando's face, the reason I deleted my previous tweet was because I didn't want to cause unwanted attention, but I'm just gonna leave this here. Thanks for blocking me, I'm sure that took a lot of courage x."
Verdasco faces fellow Spaniard Pablo Carreno Busta, who beat American Steve Johnson 6-4 6-4, in the last 16 on Tuesday.
Kyrgios breezed through with a 6-3 6-3 win over Fabio Fognini, setting up a last-16 match with German fourth seed Alexander Zverev.
Zverev, 20, rallied from a set down to beat Spain's David Ferrer 2-6 6-2 6-4.
South Africa's Kevin Anderson also reached the fourth round with a 4-6 6-2 6-3 win over Russia's Karen Khachanov, while Canadian teenager Denis Shapovalov upset 11th-seeded American Sam Querrey 6-4 3-6 7-5.