Hamilton & Rosberg battle could be upset by Red Bull - Vettel

Sebastian Vettel believes Red Bull could threaten Mercedes in this weekend's Singapore Grand Prix.

Vettel said Red Bull's pace in practice indicated they could interfere in the title fight between Lewis Hamilton and Nico Rosberg.

Vettel said: "I think we are closer than at previous races. We're here to fight and we have a good car this weekend."

Hamilton trails Rosberg by 22 points and admits the battle is "intense".

Sebastian Vettel was only on track for 10 minutes during second practice after engineers changed his engine

Hamilton won the last race in Monza after pressuring the German into two errors while leading.

Hamilton and Rosberg appeared closely matched throughout Friday practice as they embark on the latest chapter of their tight battle for the world championship at one of F1's most dramatic venues.

The Marina Bay track provides them with arguably the toughest challenge of the entire season, with a long, bumpy track and a race that runs almost to the maximum two-hour duration in hot and humid conditions under lights at night.

Hamilton said: "The circuit is so demanding and if you lose track time it's very hard to get it back, so to get through Friday with no problems is very good."

The tight nature of the track means that Mercedes' power advantage over Red Bull's Renault engine and the Ferrari makes less of a difference to overall performance, theoretically bringing the cars closer together.

But Rosberg said he was optimistic Mercedes, who have dominated the season when they have not run into problems, would be able to stay clear of Red Bull and Ferrari in qualifying and the race.

Nico Rosberg only managed 13th place in second practice after his qualifying simulation lap was ruined by Pastor Maldonado's heavy crash

"We've seen now in the last couple of races that Fridays are always closer so I'm hoping it's going to be the case this weekend and we will edge away a little bit," Rosberg said.

Vettel had a troubled Friday, missing all but 10 minutes of the second session because his engine needed to be changed following a failure at the end of first practice.

But on his single run he set fifth fastest time, only 0.551secs slower than pace-setter Hamilton, who was 0.133secs clear of Ferrari's Fernando Alonso and 0.3secs quicker than Vettel's team-mate Daniel Ricciardo.

Rosberg was 13th fastest after his qualifying simulation lap on the faster tyres was ruined when Pastor Maldonado crashed his Lotus.

Ricciardo has won three of the last seven races and scored only four fewer points than Rosberg, and 14 more than Hamilton, in that period.

"I saw Hamilton is about 0.3secs up the road but I have the feeling they have probably got a bit more in the bag," he said.

"It'll be tricky (to challenge Mercedes) but if we can make the tyres last and do a two-stop instead of a three-stop, it could open a few doors."

Full practice results

Singapore Grand Prix coverage details

Sunday's race will be the seventh time the Singapore GP has been held

Hamilton declared himself happy with having no problems during Friday's two practice sessions

Despite Singapore's volatile weather there has never been a wet race