Carlos Alcaraz began his campaign at the ATP Finals on Wednesday with an entertaining 6-3, 7-6 (10/8) win over Andrey Rublev, while Casper Ruud missed an early chance to reach the semi-finals after losing in straight sets to Alexander Zverev.

World number three Alcaraz comfortably defeated Rublev to lift himself from the bottom of the John Newcombe Group and looked in much better form despite struggling with fatigue and illness in the lead-up to the match.

There had been some doubt as to whether Alcaraz, who lost his opening match in straight sets to Ruud, would continue in Turin after quickly ending his training session on Tuesday, with his coach Juan Carlos Ferrero saying that he had trouble breathing.

Alcaraz, who has won the Wimbledon and French Open titles this season, cruised to the first set in 38 minutes, breaking Rublev in game seven and quickly winning the next two to go one set ahead.

And although the Spaniard had a harder time of it in the second set, he delighted the spectators with a series of spectacular shots which suggested he might be approaching his best form.

Rublev meanwhile drops to last place after losing his sixth straight Finals match, although he put up a battle and had two opportunities to win the second set.

Alcaraz winning in straight sets meant that Ruud would have reached the semi-finals with a match to spare if he had beaten two-time Finals champion Alexander Zverev in the day's late match.

However, Zverev put on a punishing display to prevail in straight sets 7-6 (7/3), 6-3 and all-but ensure a spot in the last four with his second win from as many matches.

Zverev took charge of the match in the tie-break at the end of a first set, which before then had gone entirely with serve, and the pair exchanged blows until the German broke in game eight of the second set and then served to love to win the match.

Leader Zverev can eliminate Alcaraz in his final match of the group stage on Friday, when Ruud takes on Rublev.