Stefanos Tsitsipas fired his 12th ace on match point to give Greece a 2-0 lead over Bulgaria with a comeback win against Grigor Dimitrov on Thursday at the inaugural United Cup.

The world number four recovered smoothly after losing the opening set to emerge with a 4-6, 6-2, 7-6 (7/4) victory in his season-opening match at Perth ahead of the Australian Open.

Greek teammate Despina Papamichail, ranked 158th, gave her country the opening point as she defeated a cramping Isabella Shinikova, also finishing with a fightback to win 3-6, 6-4, 6-1.

The mixed teams’ tournament features 18 countries playing in Sydney, Brisbane and Perth, with each tie consisting of two men's and two women's singles rubbers along with a mixed doubles.

Tsitsipas lifted in the final-set tiebreaker against an opponent he has now defeated in six of their seven matches.

"It was extremely close, but I'll take it," he said. "It was a good effort from my side.

"It was tough playing in the heat – I'm glad they closed the roof. I had to hang in there and fight, Grigor can compete with the best in the world.

"But I found a little gap and gave myself the chance to do something in the tiebreak."

In Sydney, American world number nine Taylor Fritz and 11th-ranked Madison Keys cruised through their opening ties against the Czech Republic to also get their seasons off to a winning start.

Fritz got his country away to a flyer with a 6-3, 6-4 victory over world number 74 Jiri Lehecka at Ken Rosewall Arena before Keys saw off Marie Bouzkova 6-4, 6-3.

The 25-year-old Fritz had a breakthrough year in 2022, beating 22-time Grand Slam champion Rafael Nadal in the final at Indian Wells to go with titles in Tokyo and Eastbourne.

He burst into the top 10 in 2022, reaching a career-high eight in October.

Fritz had too much firepower for Lehecka, facing only two break points on his way to a comfortable 75-minute win.

The American admitted he had felt some nerves before the match.

"It was the first match of the season and also playing a team event as well adds to it (the nerves)," he said.

"I think in the beginning of the match I handled it much better than maybe my opponent did. I got the early break, then kind of just held on to it."

Keys, who made the Australian Open semi-finals this year, came back from an early break of serve in both sets to see off Bouzkova.

The former US Open finalist overpowered her Czech opponent from the baseline, taking full advantage of the quick conditions on centre court.

In Brisbane action, Italy and Brazil were locked at one rubber apiece. Beatriz Haddad Maia, the Brazilian world number 15, was far too strong for Martina Trevisan, dropping just two games on her way to a 6-2, 6-0 victory.

"I worked very hard in the pre-season to be as ready as I could today," Haddad Maia said

Lorenzo Musetti levelled the tie at one-all when he downed Felipe Meligeni Alves 6-3, 6-4.