WELKOM, South Africa (Reuters) - Japan's Tohru Ukawa has claimed his first win in the MotoGP class with a sensational last lap pass at the Africa Grand Prix.

Ukawa, on a four-stroke 990cc Repsol Honda, and teammate and reigning world champion Valentino Rossi dominated the race from the start, with the Japanese rider taking the front on the final lap when Rossi went wide seven corners from the end.

"I'm just happy I won," said an elated Ukawa after the race on Sunday. "I know he made a mistake near the end but I was there to take the front.

"He (Rossi) chose a medium tire but I went for the hard one - I passed him - I'm happy.

"Something on the bike has improved everyday, the suspension the engine and I think it will get better."

Rossi, who started on pole position and was looking for his sixth straight win and his second of the year, admitted making mistakes after finishing second.

Rossi now leads the world championship standings on 45 points from Spain's Carlos Checa who has 27 points.

"On the last lap I made two mistakes," he said.

"I went into that corner too fast and under breaking I slid which allowed him (Ukawa) through. I then recovered and went passed him on the inside but I took the corner too fast and he went in front.

"But second place is good for the championship and the bike has been very good so I'm happy."

Rossi led into the first corner and held it through 18 laps on the gruelling 2.636 mile circuit.

Ukawa was always just a fraction of a second behind and when he set the fastest lap -- on lap 18 -- he set himself up to overtake the Italian.

On lap 19 he slipped by Rossi who lost time by looking over his shoulder to see where Ukawa was.

But Rossi stayed with his team mate and retook the lead on lap 25, holding it all the way through to the final lap.

Tire selection on the hot, dusty and bumpy Phakisa circuit was always going to be important and Rossi's decision to go with the medium compound may have cost him victory as he battled for grip in the final laps.

"My tire was softer on the right corners, but we knew this before the race," Rossi said. "I had a good rhythm with it in practice and was happy with it in the race.

"I started well and Tohru came with me. After five or six laps I tried to push hard but he came with me and eventually I made a mistake."

Italian Loris Capirossi completed a clean sweep of the podium for Honda, but the former 250cc world champion was well off the pace on his two-stroke 500cc bike.

"I tried my best today," Capirossi said. "The bike went as well as I wanted and I was the first two-stroke bike over the line which is good.

"But the difference between the two and four-stroke is quite big and on the faster circuits later in the year it is going to get bigger."