Dominic Thiem jolted his chances of making his first appearance in the semi-finals of the Nitto ATP Finals. The fourth seed collected his first win of the week on Wednesday, beating Spaniard Pablo Carreno Busta 6-3, 3-6, 6-4 at The O2.

The World No. 4 improved to 1-1 in London and will now play for a chance to reach the semi-finals on Friday against seventh seed David Goffin, who is also 1-1 after losing to Grigor Dimitrov 6-0, 6-2 earlier Wednesday. Dimitrov, 2-0, has won Group Pete Sampras and will play in the semi-finals on Saturday.

Thiem trails his FedEx ATP Head2Head series against Goffin 3-6, and the Belgian won both of their matches earlier this season, including on the hard courts of the Australian Open.

“I’m very happy and pleased that I get the chance to play for the semi-finals on Friday,” Thiem said. “We had some big, important matches in the past, also in the deep rounds of Slams. This one’s going to be another one. We know each other very well. He played a very good match here, one very bad match. I don’t know what to expect. I’m hoping that it’s going to be a tough match, and I’m trying everything to go through to the semis.”

Thiem had been 1-14 against Top 10 players on hard courts. But his powerful groundstrokes pushed the World No. 10 Carreno Busta enough for the Austrian to record his second career win at the Nitto ATP Finals. Thiem went 1-2 last year during his debut.

Read More: Win and They’re In: Zverev & Sock Play For SF Spot

Carreno Busta is an alternate and is taking the place of countryman Rafael Nadal, who withdrew from the season finale on Monday due to an injury to his right knee. But Carreno Busta looked very much like a player who deserved to be at the Nitto ATP Finals.

In the opening set, Thiem broke twice to take the early lead, and it looked as if fans would see another quick match on Wednesday. Thiem had never lost to Carreno Busta, owning a 4-0 record in their FedEx ATP Head2Head series, including a straight-sets win in February during the final of the clay-court Rio Open presented by Claro.

But the 26-year-old debutant from Spain stole momentum early in the second, breaking in the third game and then again in the ninth game with some old-school serving and volleying to even the match. He pumped his fist and shouted “¡Vamos!” at his box. All of a sudden, Carreno Busta was one set away from joining rare company.

Four alternates had played on the main stage since the Nitto ATP Finals moved to London in 2009. But only one had won a match. Serbian Janko Tipsarevic shocked World No. 1 and countryman Novak Djokovic in 2011.

Watch Classic Moment: Tipsarevic Stuns Djokovic In 2011 London Finale

In a back-and-forth third set, though, Thiem was the last to grab momentum, breaking for a 5-4 lead and serving out the match. After his 12^th ace on match point, he filled his cheeks and let out a long exhale – his semi-final hopes were still alive.

 Watch Full Match Replays

Typically a picture of calm, regardless of the score, Roger Federer still grapples with much of the same internal dialogues as his peers at crunch times on court. The 36-year-old conceals it better than most, but during a three-set triumph over Alexander Zverev at the Nitto ATP Finals on Tuesday, he admits he let it bubble to the surface at times.

Having already qualified for his 14th semi-final in 15 appearances at the season finale, there is less chance of much frustration showing in his final round-robin clash against No. 7 seed Marin Cilic on Thursday. Even a man playing as freely and confidently as Federer in 2017, though, needs to give himself a pep talk in the heat of battle.

“In some ways I feel like it’s been a season where playing freely has served me well. In the bigger moments, I try to smile on the inside and think, you know, like it’s all good,” Federer said after his victory over Zverev. “I was getting a bit frustrated with some of the shot selections at 5-1 in the third. I’m talking to myself, saying, ‘What am I even getting upset about? I’m leading 5-1 in the third. I’m one game away from qualifying in the semis. You want to get upset? There’s zero reason for that. I think it’s important sometimes to remind myself that it’s all good.” 

 Watch Full Match Replays

It could have been a very different scenario heading into Federer’s and Cilic’s final round-robin clash had the Croat not let slip a 3-1 advantage in the third set against Zverev, and a 3-0 third-set lead against Jack Sock. If Federer was airing his grievances with his play during a win over Zverev, Cilic’s frustrations in two narrow defeats would be understandable.

“Yeah, it is frustrating, absolutely,” Cilic said. “Definitely disappointing in both matches in that third set, being in a good position, putting myself in a good position. But, yeah, unfortunately I didn’t close it… It’s part of the sport, so I’m just going to try to regroup, get a little bit refreshed, try to play another good match.

“What makes the difference, I think with these top guys, is if you don’t take the chances, it’s one point here and there. I’m just going to try to be a little bit more stubborn in my preparations maybe for the next match, and hopefully next year.”

View FedEx ATP Head2Head matchup for the Group Boris Becker matches to be played Thursday at the Nitto ATP Finals and vote for who you think will win!
 Federer vs. Cilic | Zverev vs. Sock

Cilic has the aggressive all-round game to beat Federer, as he showed en route to his US Open title in 2014 but the Swiss is playing with a renewed confidence in 2017 as was evidenced in their Wimbledon final. There a blistered and emotional Cilic had no answers, salvaging just eight games in a lop-sided decider. His US Open upset remains the only time in eight FedEx ATP Head2Head clashes he has triumphed.

Cilic rebounded from his Wimbledon heartache to reach semi-finals in Tokyo, Shanghai and Basel leading into his fourth appearance at The O2. Federer, too, brings solid form into the tail end of the season with titles in Shanghai and an eighth Basel triumph, before his withdrawal from the Paris Rolex Masters on the eve of the Nitto ATP Finals to rest his body. 

His withdrawal from Paris ended any hope of supplanting Rafael Nadal as the year-end No. 1, but as he said following his victory over Sock, the ranking is just a bonus at this stage of his career.
“In some ways I’m happy he clinched it because he deserves it,” Federer said of Nadal finishing the year at No. 1 in the Emirates ATP Rankings. “And like this, I can focus on playing the tournament, and not having to talk about that at the same time.” With no more talk of No. 1, playing freely is clearly serving him well at The O2 this year.