PERTH, Australia -- Isabella Shinikova and Alexandar Lazarov triumphed in the first deciding mixed doubles rubber at the United Cup, saving one match point to pull off an unlikely comeback 1-6, 7-6(5), [10-6] over Elise Mertens and David Goffin and seal a 3-2 victory for Bulgaria over Belgium.

Mertens and Goffin had previously teamed up at the 2018 Hopman Cup, and were in perfect sync with each other throughout a dominant opening set that lasted just 21 minutes. From 1-0 down, they reeled off six straight games without facing a game point.

Mertens, a former WTA doubles No.1 and the reigning WTA Finals doubles champion, was particularly impressive, demonstrating superb court awareness and conjuring pinpoint lobs at will. By contrast, the Bulgarian duo proved shaky at net, and the last game of the set saw Lazarov send two smashes into the net -- the second down set point.

Shinikova and Lazarov got a foothold in the second set by breaking the Mertens serve twice, with Lazarov in particular raising his game at net. The 25-year-old sealed both deciding points on the Mertens delivery with sharp backhand volleys.

However, Mertens and Goffin maintained their grip on the contest by breaking Shinikova back both times and held match point at 5-4 on the Lazarov serve. But a putative return winner by Goffin missed by inches, and in the ensuing tiebreak a Mertens double fault put the Bulgarians in front.

Down set point, Mertens went long with a forehand -- and suddenly, it was Shinikova and Lazarov who had all the momentum going into the match tiebreak. The climax of the match saw Shinikova, who had been the weaker link to that point, hit peak form. The 31-year-old fired a backhand pass to take the first mini-break on Goffin's serve, then found a pair of searing forehands as the Bulgarians raced to an 8-1 lead.

From 9-3 up, Shinikova and Lazarov required four match points to close out the upset, sealed as Mertens ended a tentative backhand exchange with Shinikova by sending one long.

"There were a lot of nerves, for sure," Lazarov said. "After we lost the first set I wasn't really positive that we can take it to a decider, but we started playing better and better. After we won the second-set tiebreak, we just said to each other to play one point at a time, no pressure, whatever happens happens. I'm just happy we got the win at the end."

Shinikova concurred with her teammate.

"My experience in this event, it was really amazing," she said. "I have to thank [Captain] Grigor [Dimitrov] for the confidence to put me on the deciding match today. I'm really proud of myself that with Alex we play really good. We was like team, the fight experience was really good, and we fight until the end and it's really important for me."

Dimitrov also too the positives from a week in which Bulgaria, already the ranking underdogs, had needed to battle injury issues in multiple matches.

"I think it's great for the whole team, great for the nation, for each one of us, and gives us a very a nice boost of confidence for whatever is ahead of us," he said. "I'm just proud of the team, really. Happiness in the sense that everyone really delivered their efforts and their way of doing things. We really came together when we really had to."

Earlier, Mertens had given Belgium a 2-1 lead, though her 6-4, 3-6, 6-0 win over substitute Gergana Topalova wasn't all smooth sailing.

No.323-ranked Topalova was a last-minute replacement for Bulgaria's top WTA player, Viktoriya Tomova, who sustained a left calf injury in her Day 2 loss to Maria Sakkari. The 22-year-old was making her tour-level debut. She had never even played a WTA qualifying draw before, nor had she faced an opponent ranked higher than No.130.

However, Topalova had improved her year-end ranking from No.512 to No.308 over the course of 2022 and demonstrated why in an impressive performance against Mertens. She nailed a backhand winner on the second point of the match, and that shot would prove an impressive weapon across the 1-hour, 55-minute contest.

Hot Shot: Topalova pulls off a perfect pass against Mertens

Nonetheless, WTA Finals doubles champion Mertens' experience and all-court prowess kept her in front. The Belgian won 16 out of 22 points at net, and racked up 22 winners in total. However, her service level fluctuated somewhat; Mertens accumulated eight double faults, repeatedly beckoning Topalova back into the match.

But No.29-ranked Mertens was able to withstand her opponent's comeback attempts each time. She was twice up a break in the first set, but after being pegged back to 4-4 found her best returns and serves to take the last two games. And after Topalova came from 2-0 down in the second set to level the match, Mertens responded with a dominant third set in which she only conceded three points on serve.

"It's very fun to play in a mixed competition, but it was a little bit of a struggle," said Mertens afterwards. "I was mentally there in the third, and that was the key to win this one. First matches are always difficult, but I just tried to go for it and change the plan. [Captain] Kirsten [Flipkens] gave me a little pep talk before the third set, and that really helped."

Flipkens, Dimitrov stepping up as playing captains at United Cup

Bulgaria's Dimitar Kuzmanov showed no sign of nerves in his must-win match against Belgian ATP No.129 Zizou Bergs as he charged to a 6-2, 6-0 victory to level the Group A clash at 2-2. The 29-year-old combined consistency with brilliance at key moments to overwhelm his 23-year-old opponent.

"I was feeling really good with the conditions," Kuzmanov later told "I already played one match, and I was kind of feeling the rhythm, the ball, the atmosphere, the weather, the court, everything, I was feeling really comfortable."

ATP No. 196 Kuzmanov converted five of seven break points during a 1-hour, 27-minute win in which he appeared the physically superior of the two players. He cited his preparations for the United Cup as one of the reasons behind his strong singles performances in Perth, where he has now lost just four games in two matches.

"I came here really early," Kuzmanov said. "I was the first player to arrive for the United Cup 10 days ago to get used to the conditions. It was pretty hot when I came and now when the temperatures are not that high I am feeling really good physically on court. That's giving me really a lot of confidence and I showed that and the score showed that."

The ATP Tour contributed to this report.