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Last year's finalist Fritz played the tennis of his life, as both men served up the best encounter of the season-ending championships so far, but the American ran out of steam as Alcaraz turned on the style and took the deciding set with ease.

Alcaraz and Fritz had won their opening Jimmy Connors Group matches, and the Spaniard looked in real trouble when the American took the opening set in a tiebreak. Alcaraz faced break points in the second set but rallied back to draw level.

Fritz began to tire, and Alcaraz broke to lead 4-2 in the final set before wrapping up the win in two hours and 48 minutes by serving out to love.

"It was pretty tight, I was struggling more than him in the first set," Alcaraz said. "I wasn't serving well, and I think he was pretty comfortable from the baseline, from everywhere."

Alcaraz will be assured of a semi-final place if Alex de Minaur defeats Lorenzo Musetti in the evening match. He also needs one more victory to guarantee the year-ending world number one spot, and will face Musetti on Thursday.

"I try not to think about it to be honest," Alcaraz said.

"Obviously it's going to be a really big match for me. I'll try to control the emotions, to control myself."

The match was a battle from the opening game, with Fritz taking nine minutes to hold after hitting three aces but also facing two break points. The American forced three break points in the next game before the pair traded breaks.

Fritz raced into a 5-2 lead in the tiebreak and smashed two aces to take the set. At 2-2 in the second, Alcaraz was rattled, his drop shots which earlier beat Fritz began to fall short but, after losing advantage five times, the Spaniard held on.

Alcaraz got lucky with a shot that hit the net but crept over in the final game of the second set and broke to take the match to a third, where Alcaraz outclassed an exhausted Fritz.

"I was really relieved after the win because of everything I went through during the match," Alcaraz said.

"I wasn't feeling the ball as well as I was in the first round, but I'm really happy that I found a way to come back."