Alexander Zverev has led Germany into the United Cup semi-finals after a dominant performance on the singles and doubles court Friday night in Sydney.

After claiming a straight-sets win over Stefanos Tsitsipas to level the tie with Greece, the World No. 7 teamed with the fresh and energetic Laura Siegemund to clinch the decisive mixed doubles tie 6-3, 6-3 over Petros Tsitsipas and Maria Sakkari. The Germans will play host nation Australia Saturday night in the semi-finals.

Zverev has won five of his six matches this week, only dropping a 12-10 match tie-break with Angelique Kerber in the German's 2-1 loss in group play to France.

Siegemund, who sat on the bench throughout the tournament coming into the quarter-finals, seized on her first playing opportunity, complementing Zverev's power with the doubles nouse that has taken her to World No. 5 and 14 doubles titles, including the 2020 US Open.

“A few days ago I wasn’t sure if we would be in the quarter-finals or not. I told the team I felt we deserved to be here,” Zverev said of the team, which didn’t win its group but advanced as the the non-group winner with the best record in Sydney.

Siegemund said, “I really just tried to focus on my job. It was a pretty big match with the weight on our shoulders trying to carry the team forward to the semis. We’re a dangerous mixed when it comes to 1-all.”

Earlier in the evening Zverev sent the tie to a deciding mixed doubles after claiming a critical win over Stefanos Tsitsipas in front of a raucous capacity crowd inside Ken Rosewall Arena.

Zverev was in danger of squandering a commanding lead late in the first set but recovered and went on to close out the match 6-4, 6-4 in a result that levelled the tie after Sakkari’s big 6-0, 6-3 win over Angelique Kerber.

“I had to come out aggressive because I knew Stefanos is one of the best aggressive players, if not the best aggressive player, in the game,” Zverev said. “If he is on the front foot it’s extremely difficult to play against him and that’s why I don’t have a great record against him.”

Zverev, who narrowed his Lexus ATP Head2Head series deficit with the World No. 6 to 5-9, improved to 3-0 at this season’s United Cup after come-from-behind wins over Lorenzo Sonego and Adrian Mannarino in group play.

The 26-year-old World No. 7 was in full control of the match when he led 5-3 and had 0/40 on Tsitsipas’ serve in the first set. But after watching his fellow Nitto ATP Finals champion rally to hold serve, Zverev soon found himself down 15/40.

But Tsitsipas could not convert the lead and later in the second set made four loose unforced errors when serving at 4-all to give Zverev a leg-up to victory.

Tsitsipas sat out the singles in Greece’s first tie due to a back injury, but showed little physical discomfort against Zverev as he engaged in athletic, heavy groundstroke battles as both players covered plenty of court. But his undoing was making 24 unforced errors to Zverev’s nine.

Earlier in the evening, World No.8 Sakkari raced past former No. 1 Kerber 6-0, 6-3 to give Greece a 1-0 lead over Germany in the United Cup quarter-finals in Sydney on Friday.

Sakkari needed 73 minutes to earn her second career win over Kerber, closing her head-to-head against the German to 4-2. Both of Sakkari's wins have come on Australian soil, with her first coming at the 2021 Grampians Trophy.

“I was pretty clinical in the first set and through 3-0 in the second; I was very solid from the baseline doing what I had to do,” Sakkari said. “I knew Angie was going to find a way to come back and that she wasn’t going to give up.

“After the first set I said there was no way the match will go super easy. I’m very happy to get the first point for my team.”

Sakkari dominated both sides of the ball from the first point to the last. Serving at 67 per cent in the first set, she won 64 per cent of her first serve points and an impressive 71 per cent on her second. She wiped out the sole break point she faced.

On return, Sakkari broke Kerber at will. The German was playing in just her third singles match in 18 months, having returned to competition this week for the first time since 2022 Wimbledon. Kerber won just 46 per cent of her first-serve points and 27 per cent of her second-serve points.

Sakkari ran off the first nine games of the match before Kerber got on the board with a hold of serve at 6-0, 3-1. That hold of serve injected a surge of intensity and energy into the four-time major champion. With a searing forehand return winner, Kerber roared and got back on serve.

Sakkari promptly slammed the door on Kerber's comeback. With the German serving at 3-2, 30-all, Sakkari took control of the pivotal point with a precision backhand down the line and then closed out the break with corner-to-corner defense. She closed out the victory three games later to remain undefeated at this year's United Cup.