On Friday, the final day of round-robin matches at the Barclays ATP World Tour Finals, there are still six scenarios by which two of three players in Group John McEnroe can make the semifinals. One of them -- wins by Stan Wawrinka and Kei Nishikori in two and three sets, respectively -- would actually send No. 1-ranked Andy Murray, author of a 21-match win streak, home to the London suburbs.

Over in Group Ivan Lendl on Thursday night, it wasn't nearly as complicated. When No. 4 Milos Raonic met No. 9 Dominic Thiem, it was win and in.

At this year-end tournament missing Roger Federer, Rafael Nadal, Tomas Berdych and David Ferrer, here was a glimpse into the future of men's tennis, a battle between the two youngest qualifiers in London, a pair of heavy hitters destined to be top-10 players for quite some time.

Raonic, 25, had just one more Finals appearance on his résumé than the first-time Thiem, 23. That experience -- and a nearly unreturnable serve -- manifested itself in a sparkling 7-6 (5), 6-3 victory.

Behind 14 aces, Milos Raonic beat Dominic Thiem and made his first semifinal of the ATP World Tour Finals. Panoramic/Icon Sportswire)

In the pivotal first-set tiebreaker, Raonic hit aces on his final three service swings.

He's also had some high-powered consultants watching from his box. In addition to primary coach Riccardo Piatti, Carlos Moya and John McEnroe sat side-by-side. That's a total of eight Grand Slam singles titles.

"That's why you have three guys," Raonic joked afterward. "If the first two aren't right, maybe the other one will be."

McEnroe was brought on for a stint during the grass-court season, and Raonic responded by reaching his first Grand Slam final. McEnroe's mantra was to encourage Raonic to come forward more often to take advantage of his huge serve and wingspan. While the quickness of the O2 Arena court didn't allow an excessive amount of that, the conditions suited Raonic. Thiem, who doesn't quite have Raonic's reach, is more comfortable on clay.

And now the possibility of a Sunday final showdown between Murray and Novak Djokovic, with the No. 1 ranking on the line, looms large.

Earlier Thursday, the No. 2 Djokovic dismantled alternate David Goffin, playing for injured Gael Monfils (rib), 6-1, 6-2 in 70 minutes. Djokovic awaits the second-place finisher in the McEnroe Group in Saturday's semifinals.

By defeating Wawrinka in the Friday afternoon session, Murray will advance to the semifinals for the first time in four years. That (and four of the other scenarios) would land him a meeting with Raonic, against whom he is 8-3, including 5-0 in 2016.

The Canadian is playing the best tennis of his life, relying on one of the best serves on the planet. Against Thiem, he won 86 percent of his first-serve points, collected 13 aces and did not face a break point.

"It's a great match for me," Raonic said afterward in an on-court interview. "It wasn't a round-robin match for us; it was a quarterfinal. I was fortunate to get through today."

Thiem, whose forehand is one of the tour's heaviest and whose one-handed backhand is lovely to behold, looked a little weary. After playing a terrific first set, he was broken on his first service game in the second. Thiem's last shot was a weak backhand into the net on Raonic's second match-point opportunity.

This was Thiem's 82nd match of the year, second only to Murray's 84. He was trying to become the youngest semifinalist in London since Juan Martin del Potro in 2009, but he finished with 57 match wins.

After the then-22-year-old Thiem beat 19-year-old Alexander Zverev in the fourth round of the French Open this spring, he concluded, "Such a great player, Sascha, and, yeah, I think the difference today was maybe probably the three years age difference."

That also was the case with Raonic, who had lost his only two previous matches in London, to Federer and Murray in 2014, before pulling out with a thigh injury. Here he's won two of three matches (also beating Monfils) and pushed Djokovic into two tiebreakers. Raonic led in both frames but failed to hold his serve -- and his nerve.

"Those [key] moments he stepped up and played well," Raonic explained afterward. "I just maybe hesitated a little bit too much."

In Saturday's semifinal, most likely against Murray, Raonic will have to fly into those momentous occasions at full speed.