Italian 19-year-old Sinner is a fast-rising new star of the men's game and now has a ranking to match his age. However, he was put in his place by Novak Djokovic last week in Monte Carlo, before the world number one was himself dealt a jolting defeat by Dan Evans.
Sinner sped past Belarusian Egor Gerasimov in the first round in Barcelona on Tuesday, winning 6-3 6-2, and it will be wily world number 11 Bautista Agut who stands in his way of going deeper into the tournament.
In two meetings on hard courts this season, Sinner has edged out 33-year-old Spaniard Bautista Agut in tight deciding sets, and now they face a reunion on clay.
"He lost in Dubai against me, he lost in Miami against me, now we play once more here, so it's quite a small period of time," said Sinner.
"We're playing now our third match already, so it's going to be a very tough match. He is very, very solid. I never played against him on clay. I'm trying to be ready in the best possible way."
Bautista Agut barged past fellow Spanish player Pablo Andujar, scoring a 6-4 6-0 win.
Sinner was proud to enter the ATP top 20 this week but sees it as just a step on his journey, saying: "Obviously it's a good number, but for me at the moment not that important."
He added, quoted on the ATP website: "[I'm] just trying to improve day after day with my team and trying to improve as a player and the ranking is what it is. I'm happy to be top 20 player but the road is long so a lot of work to do."
Albert Ramos-Vinolas, Lorenzo Musetti and Frances Tiafoe, who fought off 17-year-old Spanish prospect Carlos Alcaraz, were also among Tuesday's winners in Barcelona.
Star turn Djokovic begins his campaign at the Serbia Open on Wednesday when he tackles Kwon Soon-woo, with the 18-time grand slam winner having received a first-round bye.
Tuesday's play in Belgrade saw wins for, among others, Federico Delbonis, Aljaz Bedene and Miomir Kecmanovic.
Thiem has been out of action since suffering a right wrist injury and the former world number three was able to claim the second set in his comeback match.
But he could not avoid a 6-3 3-6 6-4 defeat in a contest that lasted two hours and 35 minutes.
Speaking afterwards, Millman said: "It is great to be back here. I was looking forward to coming back and [playing]. All credit to Domi.
"I can't claim it was one of my biggest wins because he is just coming back from injury and I know how hard that is, coming back from three surgeries myself.
"I know it has been tough for Domi but the game is better having him back. Especially on this surface. I have got to take this win because when he gets better and fitter, it is going to be tough."
Next up for the Australian is Miomir Kecmanovic.
NextGen star Jiri Lehecka booked a meeting with second seed Andrey Rublev by beating Henri Laaksonen, while Filip Krajinovic overcame David Goffin.
At the Barcelona Open, fourth seed Cameron Norrie had to come back from a set down to see off Egor Gerasimov, though his 12th seeded fellow Briton Dan Evans lost to Lorenzo Musetti.
There were routine wins for second seed Casper Ruud, sixth seed Diego Schwartzman and 11th seed Lorenzo Sonego.
With his 20th birthday approaching on May 5, world number two Alcaraz will have the chance on Sunday to land his ninth ATP title as a teenager.
Alcaraz crushed the hopes of first-time Barcelona semi-finalist Dan Evans, racing to a 6-2 6-2 victory over a player who was seeking to become the first British finalist at this tournament.
Earlier, Tsitsipas, who had not dropped serve en route to the semi-finals, had to dig deep for a 6-4 5-7 6-3 victory against Lorenzo Musetti, the Italian who beat Novak Djokovic in Monte Carlo last week.
Greek star Tsitsipas bizarrely has a 0-9 record in finals at ATP 500 level, the tier at which this tournament sits, despite having won two of his six title matches in elite ATP 1000 events.
Alcaraz will start as a firm favourite in the final. He holds a 3-0 career head-to-head lead over Tsitsipas from their past meetings, including a three-set win in the Barcelona quarter-finals last year.
That was a stormy encounter, with Alcaraz taking exception to almost being hit by a shot from Tsitsipas at the end of the opening set, so the rematch could be one to savour.
"It's so special to play a final here in Barcelona, a tournament I love playing," Alcaraz said on Prime Video. "I came to watch this tournament when I was very, very young, and I played for this club when I was 12, 11. I love this club, I love this tournament, and it's so special to have the chance to lift the trophy here again.
"Last year [against Tsitsipas] we had a spicy match, but I know he's a really nice guy off the court."
At the BMW Open in Munich, it was much the same story on Saturday.
Defending champion Holger Rune, another 19-year-old with a birthday approaching, in his case on April 29, earned another shot at the title.
Rune set up a Sunday showdown against Botic van de Zandschulp, in what will be a repeat of last year's final.
A resilient Rune lived up to his top seeding by ending Christopher O'Connell's fine run, which included a win against Alexander Zverev at the last-16 stage.
The 6-3 6-2 outcome masked the fact Rune saved eight break points as unseeded world number 82 O'Connell threatened to make it a tighter contest on the scoreboard than it turned out to be.
Van de Zandschulp then scored a fine win in the second semi-final as the Dutch fourth seed got the better of American second seed Taylor Fritz, winning 6-4 7-6 (7-2)
Last year's trophy match ended in disappointment when Van de Zandschulp retired due to chest pains during the first set, with Rune saying it was "probably the worst way to win a final".
Rune has three ATP titles to his name, with 27-year-old Van de Zandschulp yet to lift a singles trophy.
Stefanos Tsitsipas breezed into the third round of the Barcelona Open with a straight-sets win over Pedro Cachin on Wednesday. Tsitsipas took 79 minutes to see off Argentine Pedro Cachin 6-4 6-2 on Pista Rafa Nadal. The second seed from Greece did not face a break point and won all three that he fashioned to set up a meeting with Canadian Denis Shapovalov, who got the better of Jozef Kovalik 6-4, 6-3. Jannik Sinner, the fourth seed from Italy, eased to a 6-2 6-4 win over Diego Schwartzman at the Real Club de Tennis Barcelona-1899. Cameron Norrie and Lorenzo Musetti will do battle in round three after defeating Pavel Kotov and Jason Kubler respectively in straight sets. Karen Khachanov beat Nicolas Jarry 6-4 6-4, while Grigor Dimitrov, Dan Evans, Alex de Minaur, Yoshihito Nishioka and Francisco Cerundolo also celebrated midweek wins. Rain hit the schedule hard at the BMW Open, where Aslan Karatsev was leading Daniel Altmaie 3-0 in the third set when play was suspended in the only singles action of the day.
The WTA Tour's top-ranked player earned $120,150 for winning the title at the Stuttgart Open last week, plus a Porsche sports car, when she beat Australian Open champion Aryna Sabalenka in the final.
Yet for taking the title at the Barcelona Open in the same week, a cheque of €477,795 ($527,000) went to Carlos Alcaraz.
Those tournaments are considered comparable in terms of status and the quality of their fields, yet the difference between the money on offer was striking.
It offered a reminder of the gap between the elite men and women at regular tournaments, despite grand slam events offering equal rewards.
Swiatek and Alcaraz both compete this week at the Madrid Open, where prize money for men and women is equal, the champion of each event collecting €1,105,265 ($1.2m).
Asked about last week's situation, Swiatek said: "Well, it's kind of obvious what my opinion is, because tennis is one of the sports where we speak about equality. I think it's better than most sports anyway.
"But still, there is a lot we can work on in terms of getting equal prize money on some WTA tournaments compared to ATP on the same level.
"Grand slams are already even, as we know. That's nice, but for sure it would be good if WTA would focus on that, but I don't really want to get into that, because it's a lot of business and sometimes politics.
"I don't think I have a lot of influence. I just can say that it would be nice for our sport if it was equal, especially because we kind of do the same work."
The 21-year-old Polish player defended the women's game as she said: "I also get people who are saying that men's tennis is nicer to watch and guys can do more because they are physically and biologically stronger.
"I think there were a lot of people, for example a couple of years ago, who were saying that WTA is not consistent and that's a shame and it should be better, but right now basically I think we are even more consistent than the guys with our game.
"Watching women's tennis gives the same emotions, and sometimes even more emotions, because we are women and we are a little bit more emotional. But, yeah, I think it would be nice if WTA could make it even."
