LONDON — Novak Djokovic prepared for Saturday's last-four clash with Kevin Anderson at the ATP Finals in ominous fashion by outlasting Marin Cilic 7-6 (9-7), 6-2 as Alexander Zverev geared up to face Roger Federer.

The Serbian world No. 1 started Friday's evening match at London's O2 Arena with the luxury of knowing he had already qualified as the winner of the Guga Kuerten Group but looked sharp and focused from the first point.

Earlier, Zverev saw off John Isner 7-6 (7-5), 6-3 to reach the semifinals of the season finale for the first time, meaning the dream Djokovic-Federer final remains a possibility.

Djokovic, dressed all in black, won a remarkable 31 points in a row on his serve from the third game of the first set until the fourth game of the second against a determined Cilic, who was unable to progress even with a victory.

"Obviously I knew before the match that I had already qualified, and I knew who I'm playing, so it wasn't easy to approach this match with 100 percent willingness to win it," said Djokovic.

"But this is a unique format and both of us wanted to win from the first point. It was a close first set which probably decided the match.

"(Anderson) is playing some great tennis, it's definitely his best season and he's continued doing that throughout this tournament. He's playing well, serving well, very aggressive, so I know what to expect."

n a high-quality contest, the 14-time Grand Slam champion stayed calm to save a set point in the first-set tie-break before taking it 9-7 on his second set point.

In the second set, Djokovic immediately put Cilic's serve under severe pressure. The Croatian former US Open champion dug deep to stay in the contest in the early stages but cracked in the fifth game, giving himself a mountain to climb and also succumbed in his next service game, leaving Djokovic to serve out for the win.

Djokovic, who has not dropped a single set in his three round-robin matches in London, has enjoyed a remarkable renaissance in the second half of the season after elbow surgery and indifferent form earlier in the year.

He has lost just two matches since the start of July, winning Wimbledon and the US Open along the way and emerging once more as the man to beat as he attempts to pull level with Federer on six ATP Finals titles.

Zverev was aware that a win by any scoreline against American giant Isner would take him through to the semifinals.

The turning point came at the end of the 12th game, when the 21-year-old German saved a set point on his own serve with an ace, going on to force a tie-break, which he won.

In the second set a single break of serve in the eighth game proved decisive and Zverev served out for the win. "It's obviously great getting to the semifinals," he said. "But the tournament isn't over. I am in the semifinals. There's only good opponents left. There's only the best in the world.

"I'm playing Roger tomorrow, which is going to be a very difficult but hopefully very nice match. We'll see how far I can go."

Third seed Zverev, coached by Ivan Lendl, is the youngest semifinalist at the ATP Finals since Juan Martin del Potro in 2009. Second seed Federer, chasing his 100th title, beat Anderson in straight sets to top the Lleyton Hewitt Group on Thursday but both players progressed, meaning early exits for Kei Nishikori and Dominic Thiem.

Cilic upbeat ahead

of Davis Cup final

Cilic said Friday that playing against some of the top players in the world at the ATP Finals had given him a boost ahead of next week's Davis Cup final against France.

The Croatian said he would approach the Davis Cup final, being held on clay courts in the northern French city of Lille, in good heart. "Just playing against top guys just maybe lifts your level a bit, focus-wise," he said after his defeat against Djokovic. "Playing-wise, you have to play every single point on a high level. I feel that might help me."

Cilic, 30, will be joined by Borna Coric, ranked 12th in the world, as well as Franko Skugor, Mate Pavic and Ivan Dodig at the final, which takes place from Nov. 23 to 25. It will be Croatia's third final after winning the 2005 title and losing in 2016 to Argentina.

"We are expecting great atmosphere obviously in Lille," said Cilic, who reached the Australian Open final at the start of the year. "It's going to be great. The French team always plays great at home. They're known for having great results in France. So it's going to be fun. It's going to be great to play, great excitement for both teams." — AFP