Novak Djokovic overcame an illness — and Alexander Zverev — to earn a second straight win at the ATP Finals.

Despite requiring a tissue to blow his nose between points, Djokovic beat Zverev 6-4, 6-1 on Wednesday to close in on a spot in the semifinals at the season-ending tournament.

"Unfortunately today hasn't been great in terms of that (illness)," Djokovic said. "Somehow I managed to gather the strength when I needed it."

The five-time champion from Serbia saved two break points at 4-4 in the opening set and then broke Zverev in the following game to take the lead.

Zverev won only one more game.

"Maybe he was a little bit sick or something like that," Zverev said. "But he still played like he felt at his absolute best. That's kind of all that matters."

The only result from Wednesday's late match that can prevent Djokovic from advancing would be a straight-set victory for Marin Cilic over John Isner.

After defeating Cilic in his opening match, Zverev still has a chance to become the first German since 2003 to advance to the semifinals of the season-ending competition.

The top-ranked Djokovic and third-seeded Zverev engaged in a number of thrilling exchanges early on, but it wasn't until the ninth game that Zverev earned the first break point of the match, aided by a friendly net cord. Djokovic saved it with a service winner but then double-faulted to give the 21-year-old another opportunity.

Zverev played a great defensive forehand slice to set up a chance to finish the point with a backhand lob, but his attempt drifted just wide and Djokovic held on for 5-4.

Zverev then made a spate of errors in the next game to bring up set point for Djokovic, and double-faulted to hand the Serb the lead.

"If I made the break, it goes the other direction a little bit," Zverev said.

The German rallied from 0-30 down in his next service game to hold, but that was the last of his resistance.

Djokovic broke for a 3-1 lead and then lost just one more point to ensure all seven matches at the tournament have ended in straight sets.

"I guess winning the first set really counts this year," Djokovic said. "Then, I guess, you have mentally quite an edge."

Victory at the O2 Arena would cap a remarkable season for Djokovic, who has already secured the year-end No. 1 ranking after dropping as low as 22nd in June.

The 31-year-old ended a two-year Grand Slam title drought by winning Wimbledon in July before collecting his 14th major trophy at the U.S. Open two months later.