There is good news and bad news for Roger Federer as he prepares for his second round-robin match at the Nitto ATP Finals on Tuesday.
The good news? Victory will get him back on track after his opening-match defeat by Kei Nishikori. And the bad news: His opponent, Dominic Thiem, has enjoyed a breakthrough year and is not intimidated by anyone's reputation.
The big question is, which Federer will show up? The one who won his 20th Grand Slam title at the Australian Open in January, or the one who looked totally out of sorts -- and dare we say it, his 37 years of age -- in his loss to Nishikori his first time out on Sunday?
And even if Federer plays well, there is no guarantee he will beat Thiem, who has won two of their three previous battles. Granted, all three meetings came in 2016 and one of the two Thiem wins came on clay, the 25-year-old's favorite surface.
This is Thiem's third consecutive appearance in the season-ending event, and for the third time, he lost his opening match, beaten 6-3, 7-6 by Kevin Anderson of South Africa.
But the Austrian had set points to take that match to a decider, and if Federer is looking for omens, he might want to ignore the fact that Thiem has won his second match at the O2 on both of his previous visits. More pertinently, Thiem will not lack for confidence after the best 12 months of his career, topped by reaching his first Grand Slam final at the French Open in June.
On the day, he was beaten by Rafael Nadal as the Spaniard won his 11th Roland Garros title, but that experience gave Thiem a massive boost, something he has carried over from clay to hard courts this season.
"Even though I lost the final quite easy, I was very, very close to my absolute goal since I was a little boy," Thiem told ESPN.com in an interview at London's O2, this week's venue. "In the final, I didn't play my best tennis, but I didn't play bad at all.
"I said to myself there is a reason why Rafa won that tournament 11 times. I was not the first one to lose against him there and maybe also not the last one.
"I learned that I was super motivated from that point to be able to live that moment again, to play a Grand Slam final and be again that close to such a big goal, which I had set up from the very beginning of my career."
In the past, standing deep behind the baseline to return serves left Thiem vulnerable to big hitters on faster courts. Now, with his return position much tighter to the baseline, he is better able to hold his own.
That improvement was clear at the US Open, where he reached the quarterfinals and was denied only in a final-set tiebreak by Nadal. Then he won the title in St. Petersburg and reached the semifinals of the Paris Masters, nice form to take into London.
"I think serve and return are now the most important shots in men's tennis, and especially on the return, where I always had my troubles, I did quite a big improvement, I think," he said. "You can see this in the results."
There is one caveat: the Federer serve.
"I think it's almost unreadable," Thiem said. "It can happen [when] you play a set against him where he plays his service games in less than one minute. You have just no chance, and that puts such a big pressure on your service games.
"The thing is that if he feels comfortable on his own service games, then he tries stuff out in the return games and he plays more freely and he is just the best ever, where he hits winners from every position and he can make you look very bad.
"You feel rushed because you're dying to get some returns in play, to get maybe a little chance to get close to a break point, but sometimes it happens that you don't even get close to that and then I go nuts."
If the real Federer shows up on Tuesday, Thiem will have his hands full, but he has proved himself to be a resilient performer.
"I consider myself mentally strong," he said. "I could be stronger, in every way of course, but it is a sign that I qualified here for the third time in a row, there is a consistency.
"Even if I have really bad losses or shocking losses, I still come back a while after to play good tennis again, and I think that's a sign of mental strength."
We'll see on Tuesday whether that's enough to see off Federer.