If Vettel wins the Japanese Grand Prix and Alonso finishes ninth or lower, then Vettel will clinch the title, regardless of what happens during the remainder of the season.
Vettel posted the quickest lap overall in Friday's practice sessions, doing so in 1 minute, 33.852 seconds. He has won three of the last four Japanese GPs (2009-10 and last year).
"The car balance is decent, but I think we can still improve," Vettel said. "Looking after the tires will be important here, especially during the race. The car works fine at the moment, but we need to make sure we set it up the right way. It's good to see we are up there today, but it's not always so important. We need to step up our game for (Saturday), as Mercedes will be strong in qualifying."
Vettel also has won the pole for the last three grand prix. He started second in Germany, Hungary and Belgium. Lewis Hamilton from Mercedes started on the pole and won the Hungarian GP. Hamilton and teammate Nico Rosberg gave Mercedes pole victories in eight of the first 11 grand prix this season.
"I think it's right to say that we had a very strong car from the beginning of the season, strong enough to always finish in a very, very strong position on Sunday afternoon," Vettel said. "Lately, I think we've been very strong in qualifying as well, whereas at the beginning of the year Mercedes definitely seemed to have the upper hand."
In 2011, Vettel clinched his second straight F1 championship with a third- place finish in Japan. The German secured the title with four races to go in that season.
Can he secure the championship again in Japan?
"The likelihood of winning it in Japan is extremely low," Red Bull team principal Christian Horner said. "When does Fernando Alonso ever finish ninth? But so long as we do win it, I don't care where that is.
"Seb is driving supremely well at the moment. The confidence he is carrying in the car and the discipline with which he is driving is remarkable. He is on a great run at the moment."
Suzuka has hosted all but four of the 28 previous Japanese GPs. The race was held at Fuji Speedway from 1976-77 and 2007-08. Many F1 drivers, including Vettel, consider this 3.608-mile, 18-turn course to be their favorite track on the calendar.
"I think Suzuka is one of the best, if not the best, in my point of view," Vettel said. "Just going through the first sector is fantastic, with the high- speed corners. It's a track where we really get to challenge ourselves, get to push the cars to their limits, and obviously, it's much more intense feeling the car at the limit in a high-speed corner rather than in medium or low-speed (corners). There are quite many around here. It's a very tricky track, very challenging."
After Japan, F1 moves on to India (Oct. 27), then Abu Dhabi (Nov. 3) and Austin, Texas (Nov. 17) before the season wraps up in Brazil (Nov. 24).
Series: Formula One. Date: Sunday, Oct. 13. Race: Japanese Grand Prix. Site: Suzuka Circuit. Track: 3.608-mile (5.807-kilometer), 18-turn road course. Start Time: 2 a.m. ET. Laps: 53. Miles: 191.117 (307.771 km). 2012 Winner: Sebastian Vettel. Television: NBC Sports Network.